How do I build a dynamic URL with a merge field, save it in a hidden field on the form?

We are trying to dynamically build a URL using a merge field associated to the contact to redirect them to a personalized lander. The instructions given to us was to use the hidden field as the redirect in the processing step. We need to build Javascript to build the URL and then have it populate in the hidden field. We have been successful in building the URL 2 different ways, one being from KB (https://support.oracle.com/cloud/faces/DocumentDisplay?_afrLoop=303208761315581&_afrWindowMode=0&_adf.ctrl-state=mig9uddf_193). Successfully created the hidden field on the form and tied the HTML Name from the above code to the field. When we process the landing page, we can see the dynamic URL in the source but it does not populate the field for the redirect to use. We are looking for options and ideas on what we may be missing on the set up or js.