Control redirect after account creation

I have a registration form on a staging site – assume the URL is something like /register/. Other sites also link to this form with a source parameter, for example /register/?source=gym.

All 4 sites are connected using the Remote Users Sync plugin. Let’s call them:

staging27 (this site has the form), staging56, staging57 and staging8.

The issue I have is that whenever a user registers, they are redirected to the My Account page on staging56. I think this is because this is the first site in the Remote Sites list.

I feel like there must be a way to control which site the user is sent to after registering, but I can’t find it. Can you point me in the right direction, please?

The page I need help with: [log in to see the link]