Please, follow the steps to create a popup inside popup:
1. Create any popup, except Iframe, Image and Video types (our example is with HTML popup)
2. Enter a Title for your popup (visible only to Admin)
3. Enter a "click text" inside your popup 1, it will open the popup 2 when clicked on it
4. Highlight the "click text"
5. Click "Insert popup" above
4. Select the popup you need to be opened from the 1st one
5. Now it's inserted in your Popup 1
6. Save the changes and insert it on the page you wish!
Here you can see the result of the Popup 1 triggering the Popup 2.