Stadia's kind of in Early Access mode right now - only users who have purchased a hardware bundle are able to get access codes for now. That should change in the near future. In fact, all those with a Founder's or Premiere bundle will get a buddy code they can share with a friend in about two weeks which will provide access without requiring a hardware purchase.
Maybe someone here will hook you up with that pass once they get it?
You should be able to just sign up for Stadia Base (for free) or Stadia Pro (for the monthly fee) without purchasing any hardware by early 2020. I would guess that the full general roll out of the hardware-free accounts will happen sometime before the 3 months of Stadia Pro included with the kits expires.