The button, and it's location is your decision. With my Mico I didn't have to hold the button, so it was mounted where convenient, in my case on the shifter riser.
With the new summit line lock,, I will need a switch on the shifter, because I shift fron second to third in the water. My other cars used a button on the shifter knob
(The big giant knob button is usually used with a transbrake, in my experience. )