Create an executable file with the same name as the action name. The content of the file does not matter.
For example, if the action definition is:
ACTION MyFavoriteApp { EXEC_STRING Mfa -file %Arg_1% DESCRIPTION Runs MyFavoriteApp ICON Mfapp }
then the action file would be an executable file named MyFavoriteApp. In File Manager and Application Manager, the MyFavoriteApp file would use the icon image Mfapp.size.type. Double-clicking MyFavoriteApp's icon would run the action's execution string, and the icon's On Item help would be the contents of the DESCRIPTION field (“runs MyFavoriteApp”).