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 😀