Hi there,
so i tracked this down in the Hustle Docs:
https://wpmudev.com/docs/wpmu-dev-plugins/hustle/#click
its where you can set the Trigger ( to open the Pop Up ) using a Click event.
So you won’t want: Render a trigger button
Instead you will only want” Click on existing element and in the CSS Selector(s) field provided you can add your own class eg. open-popup
Now if you use GenerateBlocks to add your button, with the button selected go to Advanced > Additional CSS Class(es) add your CSS selector eg. open-popup