I'm about to do this myself, it seems simple enough -- From the new button -- the bell wire to matching polarity on the GDO or existing button - the speaker wire to R1/R1 terminals on the ZEN16. Software install and setup as detailed on the Zooz site,
I am figuring a checkpoint would be after the new buttons bell wires are attached -- the button should work to open/close.
You got it. And in addition to the button, you could short the "speaker wire" together briefly to operate the door. The hardware installation is pretty trivial.
The software and setup per the Zooz instructions is a bit tedious but just take it step by step.