AFAIK it's currently not possible to assign a continuous parameter (e.g. Mixtool > Gain) to a Macro Button. Or in other words a VST Knob value to a Macro Button.
I would like to suggest a feature where this is possible. So it's possible to create a button that switches a parameter to a value when ON and switches to a different value when OFF.
Target transform for a Button might need a dialogue with two points (ON and OFF).