One, use the Default Applications Chooser. If what you want isn't shown in the relevant 'drop-down', simply type the full $PATH to it directly into the combobox.....most applicable to items where the app launcher is not is in a 'standard' location (like portable applications, although the way I add the MenuEntry for these is to merely sym-link the 'LAUNCH' script into /usr/bin with an appropriate name, then simply 'call it in the standard way from the .desktop entry).