redirect to subdomain or different domain url on submitting form

This person already knew that Drupal would refuse to redirect to URLs, not because they were merely absolute, but only if they were absolute and not in the Trusted Host Patterns.

And that was really all the information i needed, fortunately for me, although unfortunately for them they only had to state all that because something wasn’t working somehow for them:

On a form you can set where you go next with setRedirectUrl (or if you have the route, setRedirect).