Linking two surveys and passing random string identifier

I know there have been all sorts of related questions, but I still can't figure out the right way to accomplish the process I need in order to link between two surveys: 

• The first survey is an open one where people register for the research (consent form). 
• The second survey, which is the actual research survey, should be closed, and people should automatically be redirected to it only after registering via the first survey. 

I want both surveys to have a shared identifier field with a random string that connects them, but other than that, there shouldn't be any other shared fields between them. Ideally, the random field should be saved as an external data reference because it seems more organized to me, but if that's not possible, it's also fine. 
What I have tried so far: 

• On the first survey: I created a hidden field with JavaScript that generates a random sequence of letters and numbers.
• On the survey flow I created an embedded data element that take this random string and save it as "user_id".
• On the post survey option I created a contact trigger that added those people to a contact list and convert the user_id to external data reference.

And now I’m stuck...🙄

I tried to create a custom end-of-survey message that forwards to the next survey and passes the string to it, but it doesn't work as expected.
This is how the link looks:

https://qualtrics.com/jfe/form/SECOND_FORM_ID?${e://Field/user_id}

However, it only passes the literal text "user_id" instead of the actual random string.

I'm also unsure about what needs to be done in the second survey. I suppose I need to add an authenticator element at the beginning of the survey flow, but I don’t know how to make it automatically capture the string from the URL and pass the the user smoothly to the actual survey.

Any help would be appreciated.