It's easier to sign up for an Ubisoft+ subscription from the Stadia app than it is to sign up for Stadia Pro. Actually getting to a sign up page involves tapping the Gift icon in the upper left, scrolling all the way down and tapping More About Pro. There should be a sign up method made obvious in the user account menu, if not listed right above Ubisoft+, then under Purchases & Subscriptions.
I found it hard to check since I have a pro subscription, but a friend told me that you could also tap on a "pro discount" to be prompted, of course is not the best approach, did you had a previous subscription active?
I have a few separate times, yeah. I think I've always done it in a browser, though, so it surprised me how difficult it was to sign back up in the app.