thanks for making this! I'd love to use the microphone key (fn + mic) to trigger Handy but even after turning off dictation it doesn't seem like the system allows that key to be used (I get a dialog prompting me to turn on dictation).
https://karabiner-elements.pqrs.org/
There you can map (whatever key) to (whatever other key). E.g. I have right command mapped to F20, available to all other apps .
I added a issue (and comment) for this on the GitHub repo: https://github.com/cjpais/Handy/issues/47