in https://developers.hubspot.com/docs/methods/oauth2/get-access-and-refresh-tokens I read: "The grant type of the request, must be authorization_code for the initial request to get the access and refresh tokens."
Does that mean you're not providing any other OAuth2 grant types? Especially not "client credentials grant" ?
If so, is there any other means of authorization for a backend service (i.e. not on behalf of a specific user) available - besides API key?