On the Enter Results screen you can enter DFS in the Time column for a Declared False Start.
The following information is taken from the USA Swimming 2016 Rulebook.
101.1.3.E Declared false start: swimmers reporting to the Referee prior to the start of their race and declaring their intent not to compete will be disqualified except as noted in 207.11.6D(1). (page 20)
207.11.6D(1) Any swimmer qualifying for a C, B, or A (bonus and consolation final or) final race in an individual event who fails to compete in said final shall be barred from further competition for the remainder of the meet, except as noted in paragraph E. A declared false start under 101.1.3E or deliberate delay of meet under 101.1.5 is not permitted and will be regarded as a failure to compete. (page 79)
207.11.6E Exceptions for Failure to compete. (pages 79-80)
Wisconsin Swimming also has an excellent explanation in their Declared False Start paper from 2009.