You'll need to look up the song ID value for the song that you want to play. You can look it up in the xbmcSongs payload xml file, if you like.
Clone your "Play song {1}" command in your command tree.
In the new clone, revise the command name and phrase to say whatever you want (e.g. "Play my one very special song. You know the one.")
Obviously, you don't need the payload xml file or anything like that if you have one specific song that you want it to play, as the payload xml allows us to use one command to ask for multiple, similar things.
In the macro, replace the payload reference {1} with the id of the specific song that you want XBMC to play.
That should be it, depending on your ultimate goal.