FCC Part 97 Amateur Radio Service rules allow US licensees in §97.119(c) “to append to their FCC-issued call sign (before, after, or both) a special indicator of significance, so long as it does conflict with a prefix assigned as a prefix for another country.” One or more such indicators may be included with the call sign, and each indicator must be separated from the call sign by a slant bar (or, in voice communication, by any “suitable word” that denotes the slant bar, such as “stroke”). Self-assigned indicators may be included before, after, or both before and after the FCC-assigned call sign. No self-assigned indicator may conflict with any other indicator specified by the FCC Rules or with any prefix assigned to another country.

1x1 Special Event Call Signs

     When transmitting in conjunction with an event of special significance, an amateur station ("special event station") may transmit the identification announcement using a special event call sign in accord with the procedures detailed below. Substituting a special event call sign for its assigned call sign may help a special event station call attention "on-air" to its participation in the special event and to the unique opportunity for the amateur service community to exchange greetings with the station. Use of these provisions, however, must not detract from the station making the source of its transmissions known to those receiving them. The special event station must also transmit its assigned call sign at least once per hour during such operation.

     A block of 750 call signs is available for use in the special event call sign system. The format of each call sign ("one-by-one") consists of a single letter prefix "K", "N" or "W", followed by a single digit numeral "0" through "9", followed by a single letter "A" through "W" or "Y" or "Z" ("X" is not available to amateur stations). The station license grantee must obtain coordination for the use of a special event call sign through one of the following special event call sign coordinators. These coordinators maintain and disseminate world-wide a common data base for the day-to-day usage of the one-by-one format call signs (for example "K1A"). Upon completing the coordination process, the special event station may substitute the one-by-one format call sign for its assigned call sign during the period of the special event.

Use of Special Call Sign Indicators

      In addition to the special event call sign system, any amateur station, including a special event station, may include with its assigned call sign one or more indicators (example "W1AW/national convention"). Each indicator must be separated from the assigned call sign by a slant ("/") or any suitable word that denotes the slant mark ("portable," "stroke," etc.). If the indicator is self-assigned, it must be included before, after, or both before and after, the assigned call sign (example of a portable W1AW station operating the contest in Hawaii - "KH6/W1AW/contest"). No self-assigned indicator may conflict with any other indicator specified by the FCC Rules (such as "AA", "AG", "AE" or "KT") or with any prefix assigned to another country (such as "DL", "F", "G" or "VE").

FCC rules allow Amateurs to add a suffix to their call sign.
The FCC Rules do not allow amateurs to create callsigns in the field.

FCC Special Event call sign information page:
https://www.fcc.gov/wireless/bureau-divisions/mobility-division/amateur-radio-service/special-event-call-signs  

Here's call sign suffixes information in an ARRL News item: 
http://www.arrl.org/news/use-of-special-call-sign-suffixes-in-the-us-during-covid-19-pandemic 

Amateurs could also hold a special event and have a special event call sign assigned to the event. Reserve a special event call sign on the 1x1 website:  http://www.1x1callsigns.org/   
 

A block of 750 call signs is available for use in the special event call sign system. The format of each 1x1 call sign consists of a single letter K, N or W prefix followed by a single numeral, 0 through 9, followed by any single letter except the letter X, which is not available to amateur stations.

Call signs are available by call sign, by date(s) assigned for use. Call sign assignment is subject to availability, per reservations/assignments made as appearing in the 1x1 Call Sign Database.
 

Assignments are made by calendar date, for the full day at Coordinated Universal Time (UTC) or Zulu Time (Z):  0000Z to 2359Z

A 1x1 special event call sign may be reserved up to 365 days before the first date of its intended use.
 

If your intended use and purpose for obtaining (for short term use) a 1x1 Special Event Call Sign meets the assignment criteria, simply provide the requested information on our online form. The information will then be reviewed by the ARRL as a coordinator and then your application will be coordinated—by assigning for your use for the time frame requested the first call sign offered on your list of choices. Should a 1x1 call sign not be assignable to you, the ARRL will contact you via one of the methods indicated with your application.

1x1 Special Event Call Sign Assignment Criteria:

• The intention of the Amateur Radio 1x1 Call Sign system is to provide a special
  call sign for a special event operation which usually commemorates an event
  which is publicly significant (ideally, this is a one time, non-recurring celebration,
  festival, anniversary, holiday, convention, dedication, public demonstration, etc.).

• While the 1x1 Call Sign system in not intended for the exclusive ability to have a
  short call sign during an upcoming amateur radio operating event or contest,
  should such an operating event or contest timeframe fall within the same timeframe
  of an event which has been coordinated with a purpose as outlined above, the special
  event 1x1 call sign can certainly be used for any Amateur Radio operating purpose
  during the coordinated time period.

• The 1x1 call sign being sought will only be needed for 15 days, or less (unless a
  persuasive showing is made that the event timeframe is so extraordinary that
  exceeding the 15-day call sign assignment period is appropriate).

• Amateurs may not reserve multiple bordering 15 day periods. Reserving more than
  one 1x1 call sign during a calendar year is discouraged.

Apply for a 1x1 Special Event Call Sign.

Confirmation that your requested 1x1 Special Event Call Sign has been assigned is accomplished once your reservation/assignment appears in the 1x1 Call Sign data base. No hardcopy confirmation of assignment is necessary. No special license will be issued by the FCC. Simply check the 1x1 database a few days after submitting your application to confirm that the assignment has been made. Then at 12:01 AM (local time) on the first day of the assignment period you may begin use of the 1x1 call sign assigned for your event.
 

The following five Special Event 1x1 Call Sign Coordinators are presently
appointed by the FCC:

The American Radio Relay League, Inc. VEC Department 225 Main Street Newington, CT 06111-1494  Telephone 860-594-0300  Fax 860-594-0339  Email: vec@arrl.org

The W5YI VEC 2000 E. Randol Mill Road Suite 608-A Arlington, TX 76011  Telephone 1-800-669-9594  Fax 817-548-9594  Email: nb5x@w5yi.org 

Western Carolina Amateur Radio Society/VEC, Inc. 5833 Clinton Highway Suite 203 Knoxville, TN 37912-2500  Telephone 423-688-7771  Email: WCARS@KORRnet.org

Laurel Amateur Radio Club, INC. PO Box 146 Laurel, MD 20725-0146  Telephone 614-864-1884  Email: wd8qwr@twc.com   

W4VEC Volunteer Examiners Club of America PO Box 41 Lexington, NC 27293-0411  Email:  bstewart@windstream.net   
 

The 1x1 Special Event Call Sign Coordinators shall ''coordinate, maintain and disseminate a common database of amateur station special event call signs.'' The five FCC appointed Special Event Call Sign Coordinators maintain a shared 1x1 call sign database so that both requesters and coordinators will know which call signs are available at any given time.

The FCC says that special event 1x1 call signs are authorized for use "in conjunction with an event of special significance." This suggests an event with high public visibility, as opposed to a routine ham radio contest or other operating event. However, a special event call sign could be used in conjunction with a contest if one were to occur within the call sign reservation period.

The five special event call sign coordinators have the authority to determine if a request is in keeping with the intent of the special event call sign system. Any licensed amateur may request a special event call sign. Call signs may be requested up to one year in advance of their use on the air. Call signs normally will be granted for periods of 15 days or less. Because of the limited number of call signs available, requesters are encouraged to limit their requests to the duration of the special event.

A block of 750 call signs is available for use in the special event call sign system. The format of each 1x1 call sign consists of a single letter K, N or W prefix followed by a single numeral, 0 through 9, followed by any single letter except the letter X, which is not available to amateur stations.

For more information on the special event call sign system, call 860-594-0300 or e-mail vec@arrl.org. You also can contact any of the other special event call sign coordinators or visit the  1x1 Call Sign FAQ page.
 

Amateur Station Special Event Call Sign System:  When transmitting in conjunction with an event of special significance, an amateur station ("special event station") may transmit the identification announcement using a special event call sign in accord with the procedures detailed below. Substituting a special event call sign for its assigned call sign may help a special event station call attention on-air to its participation in the special event and to the unique opportunity for the amateur service community to exchange greetings with the station. Use of these provisions, however, must not detract from the station making the source of its transmissions known to those receiving them. The special event station must also transmit its assigned call sign at least once per hour during such operation.

Special event call signs:  A block of 750 call signs is available for use in the special event call sign system. The format of each call sign (one-by-one) consists of a single letter prefix "K,” "N" or "W,” followed by a single digit numeral "0" through "9,” followed by a single letter "A" through "W" or "Y" or "Z" ("X" is not available to amateur stations). The station license grantee must obtain coordination for the use of a special event call sign through one of the special event call sign coordinators. These coordinators maintain and disseminate world-wide a common data base for the day-to-day usage of the one-by-one format call signs (for example "K1A"). Upon completing the coordination process, the special event station may substitute the 1 x 1 format call sign for its assigned call sign during the period of the special event. Volunteer entities that have been certified as special event call sign coordinators can be found elsewhere on this page.

Indicators:  In addition to the special event call sign system, any amateur station, including a special event station, may include with its assigned call sign one or more indicators (example "W1AW/national convention"). Each indicator must be separated from the assigned call sign by a slant ("/") or any suitable word that denotes the slant mark ("portable," "stroke," etc.). If the indicator is self-assigned, it must be included before, after or both before and after, the assigned call sign (example "KP2/W1AW/contest"). No self-assigned indicator may conflict with any other indicator specified by the FCC Rules (such as "AA", "AG", "AE" or "KT") or with any prefix assigned to another country (such as "DL", "F", "G" or "VE").

Questions concerning 1 x 1 call signs and the common data base should be directed to the special event call sign coordinators.

If you have any comments or questions about this information, please contact FCC Wireless Telecommunications Bureau at (877) 480-3201.  Visit the FCC website or the 1 x 1 website for more information.