Hello @SCOREOC ,
The content field ‘email lists’ that is marked required. This is the list that the contact will be adding their email to once they click on the blue ‘Sign Up’ button on the popup form. The coding for that is already done for you, just once you’re done and have published your signup form it will then generates your Universal Code to be placed within your website. Full steps on placing the code within your website are linked here.
The email list is what's selected when creating the pop-up. It'll be on the side-bar in the editing screen.