Hi there!
We’re making an effort to plum a bit our backlog and wanted to update this topic for any future users ending up here:
Getting an access token for the registered API can be done with the registered machine-to-machine application credentials (client credentials exchange). It is well explained here: Call Your API Using the Client Credentials Flow
Example usage of the flow in Actions (getting / modifying access token): Machine to Machine Flow.
We also have an article on caching the Management API token in Actions - but please treat it as an outline that needs to be well-tested and updated to your needs: Caching Management API Access Tokens in Login Action
Thanks,
Marcelina