most of us have already given the buddy passes away, so it's rare to still find one, but you can also try your luck in other communities like reddit, facebook, etc. maybe someone there still has a key for you.
Otherwise the only thing you can do is
Mh yeah I heard from that promotion, so you basically should be eligible for a code I would think (otherwise it would not make sense for BT to handle those out), did you try with both of their support to get a code? Most successful could be - I imagine - to talk with Stadia support in that case. If you have opened a ticket already with Stadia support, do you have a Case-ID to share that @GraceFromGoogle can check this out?
Wish you the best though for a key!