If you'd like to add a pop-up window to your site, you will need to use external coding and embed this into your site using an HTML widget. These Google Search results offer a variety of suggestions you can use.
Once you have the HTML code, you can add this to your HTML widget.
1. Go to Widgets > Popular.
2. Drag and drop an HTML widget onto your page.
3. Paste in your code.
4. Click Save.
Please keep in mind that while the code may work now, we cannot guarantee that it will continue to work in the future. Codes can become obsolete and/or conflict with the enhancements made to provide you with the best Sitebuilder possible.