Landing Page - Javascript - URL Params

Latest Mautic 4

Trying to understand what the process is to get parameters to populate a form

Trying to build a unsubscribe landing page, where the form will populate a hidden text along with the standard name, phone and email fields which we can pass through via the email contact fields.

I have tried to add in the JS code from the gitbub below, but the JS drops from the template when added in the frontend, and the templates it doesnt show at all (suspecting I am using the wrong template - tried blank and mjml_blank)

What am I missing to get params to pass through to the mautic form
I have a value for the field ID, attributes, auto fill on etc

Missing something, all I have found online are old tutorials