Hello,

You can first edit the respective Page on your Site with the page builder plugin active on your Site > then in that use the Popup element of the Theme > then under that element options select the HTML Block accordingly: https://snipboard.io/Z1a2tr.jpg

If you want to have some Form shown on the Popup Content then you need to make a Contact Form on your Site and then use that Form under the HTML Block made under the Dashboard > HTML Blocks.

Afterwards, you can select that HTML Block created from the dropdown available in the Popup element.

Hope this helps !

Best Regards.