How to launch Fitaly with button or pass through slides

sorry if this horse has already been beaten to death, I didn't find it
in the FAQ and I'm brand new here.
I'm a dedicated user of Fitaly Stamp unless I'm working in a dark environment in which I can't see the keyboard. In these cases I'm a dedicated user of Fitaly.
It is kind of difficult to disable Fitaly Stamp when you don't see the keyboard, but this is a minor problem. Once it is disabled, how can I bring up Fitaly (the on screen version)? It seems I can't assign it to a button as this appears to be only possible with applications which Fitaly isn't per se. Any ideas how to solve this dilemma? I'd prefer using a button if that's possible at all, but if a pass through slide done from Fitaly Stamp might do the trick, this would be nearly as fine.

Thanks for any insights

Frank