Change the appearance of Marketing and Event forms using URL parameters in Customer Insights - Journeys

How to change the appearance of a Customer Insights - Journeys form based on URL parameters, for when you want to (or have to) use the same form for different personas, brands or behaviours. For example, an event will only allow you to use one form for taking registrations, but you may want to change the form visuals based on the person invited. Using separate events would make it difficult to manage capacity, impossible to use streaming in teams and some very disjointed reporting. So lets make a single form work for both!