At launch and until Stadia Base is available, the only chance to play with Stadia is to use Stadia Pro.
If you don't want to activate it - what I can understand from your point of view - then you would need to wait until Stadia Base is out (which is the free-version), but Google mentioned it's coming next year (see here).... ![]()
You can sit on your code and not activate it, but you won't be able to get into the service without it and you won't get a chance to reserve your preferred username until you do.
I say just go for it, unless you're absolutely sure your username won't be taken and you have no interest in the current batch of games included with Stadia Pro (Destiny 2 Collection and Samurai Shodown). There is no guarantee that these games will still be available to add to your account at a later time, as I think we all expect the current batch of free games with Stadia Pro to be rotated out when the next one(s) are added.