This is what I tried the 2nd time with the other 2 keys. You asked me earlier my pack. I have Patron (4 beta keys). While I have a very old account, I did not initially back the game.
Anyway, the two initial keys are irretrievable (Dual won't email the reset passwords to those "new" accounts)
2nd attempt I tried what you mention here, and sent my sons 2 more keys. This time using this field to add it to existing accounts. Got an error the key was already used or something similar. So in the end, all 4 keys have not be usable for us. I reached out over a month ago or so to support, but no response.
Thanks for replying though. Appreciate the help