If you are using Wordpress, you can create a pop-up form very easily using Boxzilla: you can download it here.
First, go to the plugin, and click on "Add new":
Give your form a title and paste the Plezi script directly into the "Text" part:
Determine the design elements of your form:
Finally, choose the rules for the appearance of the pop-up (on which page, from which action etc.):
All you have to do is publish your form, and you're done 😀