Viessmann finally opened their developer portal after announcing it in 2020!
Although they really took their time to do so, giving us the time to incoorporate the requested changes is not planned. From the EMail:
Important Changes In order to keep our API stable into the future, we must therefore shut down access to any old API keys that were not obtained through the Viessmann Developer Portal. On [July 15th, 2021](calendar:T2:July 15th, 2021) access to old API keys will no longer be possible.
Actually I’m trying to get an own API Key. My initial trials are done manually with http requests, however that doesn’t work (ATM?).
As far as I understand the OAuth2 used by Viessmann needs a “code-verifier” which isn’t part of the OAuth2 implementation of openHAB core.