If Desantis does not win nomination, or Trump does not run 3rd party, this will resolve N/A. Otherwise it will resolve YES if Desantis wins presidency, and NO if he loses.

(see also this market for if Desantis is nominee and Trump does not run 3rd party)

Updates and clarifications:
If Trump runs 3rd party but then drops out before election day, that will still be considered a run.