Let me show how to do this on the example.
1. First of all, go to the PopUp settings and on the Main tab, in the “Where to show section” choose “Click on certain link / button / other element” and copy this shortcode https://prnt.sc/fm062h.
2. Then on the Dashboard, in the Appearance tab open “Menus” https://prnt.sc/flzzgg. You will see list of your menu from the website https://prnt.sc/fm05ag.
3. Choose needed item from Menu Structure and insert the shortcode you copied in the Title Attribute field https://prnt.sc/fm0897. Now your PopUp will show by clicking on the menu item you chose.
You can find more helpful information (and PopUp on click tutorial) in this article https://supsystic.com/documentation/open-popup-click/