Hi, 

1. you need to create a popup first, using the Popups Builder (you can also import the pre-built template), kindly check the instructions https://codex-themes.com/thegem/documentation/thegem-popups-builder/ 

2. After that you need to link this popup to this button as described here https://codex-themes.com/thegem/documentation/thegem-popups-builder/ in the chapter "On click popup" https://prnt.sc/63nP5omLI7pJ 

Kindly let us know if you still will need help with this.