The favorite selection is populated dynamically during runtime. In general all options - so called StateOptions - are a list of label and value pairs (label and value are the same in Kodi binding). You can use the same item to send a command (=value) of your favorite towards Kodi:
myKodi_playfavorite.sendCommand("Favorite1")
It is possible to access the whole list of state options in a rule and work with them: