While I... enjoy.. ESO, I do not enjoy the lack of transparency regarding how to actually connect your account to Stadia.
Nowhere in the Help Article or on this website is there any indication that not only do you have to purchase it from here or Steam or wherever, you also have to buy it - Again - on Stadia to be able to play it on Stadia's buggy, constantly-lagging streaming server. And yes, my internet speed is gargantuan in terms of speed, there is no reason that servers should be lagging when everything else I stream is more than perfect.
Briefly summarizing - Do not believe that you will be able to link your account to Stadia and instantly be able to play the game. Sorry for sloppy grammar, but 'it ain't gonna happen'. And to be honest, Stadia isn't really worth the effort to go Pro.
This was made for informational purposes only. Any responses will be ignored.