Add an alternative host to an already scheduled webinar?

Hi! I would like to add an alternative host to a webinar that is already scheduled.

My primary motivation for doing this is so that this person can join the practice session I'll start before the webinar, and help me ensure that everything is working correctly. (Versus upgrading them to co-host, which I can only do once the meeting starts).

Any advice for how to do this? Or if not, another way to manage the situation? Thank you so much!