WordPress Custom Fields Update - Urgent Job

We are looking to add the attached contact form to all enquiry CTA buttons throughout a site. We're aiming to add a lightbox contact form to a button, however, the current WordPress template has been made using custom fields only allowing a button link as a URL.

The plugins, Contact Form and Popup Maker have been used. This has been used to create the contact form called 'Request A Call Back' and added this to a popup, under the same name. The pop up has been set to act as a lightbox when clicked. To action the popup it states:

Adding the class "popmake-2990" to an element will trigger it to be opened once clicked.

So, where we get stuck. As mentioned above the WordPress theme allows a button to link to a URL only, not allowing to call this popup or contact form. We need your expertise here, please!

I can supply accs to the website admin as well as FTP

Thanks