Right-click on the menu on the edited page and select "Edit menu";
Click on "New Item";
Enter the "Item Name" (for example, banner), select the radio button "Popup", enter the size, click "Apply";
Select a page from the list where you want the popup to be shown;
Check the box "Show popup when page is opened" and select the created page from the dropdown list, click "Apply";
In the upper left corner from the dropdown list select the popup page you created, edit the page and go back.