Having the same problems. Followed the documentation at https://www.bitdefender.com/support/bitdefendergravityzoneand-microsoft-windows-defenderatpintegration-1987.html. There's a bad link on the "Integrate the solutions" step on item #1 where it asks you to open the "Onboarding Page" but that link is a circular link to the article you're currently on. I've opened a support request asking them to point this to the https://www.bitdefender.com/site/Downloads/sendtokenmicrosoft/ page discussed earlier.
Hunting around for a fix myself I came across two pages where I can generate API tokens on the ATP side: Windows Defender Security Center > Settings > APIs > Threat intel or SIEM. I generated tokens in both areas and was given Access Tokens and Refresh Tokens. None of the four tokens worked on the https://www.bitdefender.com/site/Downloads/sendtokenmicrosoft/ page. Even tried plugging in the Bitdefender API key for giggles - no dice. "The provided token is invalid. Find out how to generate a valid Windows Defender ATP token in this KB article."
Doubting that Bitdefender will have a timely resolution to my support case. Any insight you guys can provide on your side? Thanks!