I recently struggled to use the Bubble API Oauth2 Custom Token system for Spotify’s Client Credentials Flow authorization flow.
I am posting the solution here in case this helps other people in similar situations.
The content type header is set to: application/x-www-form-urlencoded
The client id and client secret need to be concatenated with a colon between them, and then, converted to base 64. (NOT converted to base 64 and then concat’d as I did :P)