Logic Pro: How to assign MIDI controls to software instruments?

Hi all!

I recently learned about controller assignments in Logic, and I set up a MIDI note to toggle the mute function on a track with a synthesizer on it. Now I want to assign the stop button within the synthesizer (top right of the image) to the same MIDI note, and I can't figure out how to do it. I tried going into Learn mode and pressing the button, but it didn't work. Is there a way to assign specific controls *within* software instruments to Logic's controller assignments, rather than only Logic-specific commands?

I'd appreciate any suggestions!