Multisite subsite independent registration

I use multisite and my main site is

I am teaching a course on and I need people to be able to register directly on that website without being re-routed to

essentially I need this to work:

without it redirecting to the registration page.

I have researched this, which was not very hopeful:

and this, which seems like a solution- but is WAY over my head:

There must be an easier solution!?

Help appreciated