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 2018 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 80)
207.11.6E Exceptions for Failure to compete — No penalty shall apply for failure to withdraw or compete in an individual event if:
(1) The Referee is notified in the event of illness or injury and accepts the proof thereof.
(2) A swimmer qualifying for a C, B, or A (bonus or consolation final or) final race based upon the results of the preliminaries notifies the Referee within thirty (30) minutes after announcement of the qualifiers for that race that they may not intend to compete and further declares their final intentions within thirty (30) minutes following their last individual preliminary event.
(3) It is determined by the Referee that failure to compete is caused by circumstances beyond the control of the swimmer. (page 80)
Wisconsin Swimming also has an excellent explanation in their Declared False Start paper from 2009.