That’s not how PHP requests work, you cannot set a variable in one request, then read it in another request. It isn’t like a Node or Python application where the program runs continuously in the background, every request is a blank slate.
So, to make it work, in the first request you need to put it somewhere that the second request can read from, for which there are several options:
- Cookies
- User meta
- Options/Transients
As it’s a logged in user, and persisting across sessions would make sense, user meta seems the most logical choice, via update_user_meta and get_user_meta