401 error when adding webinar registrant, when using access token created from Server-to-Server oAuth

I am able to create an access token, using the server-to-server method.

When I use the token returned from the first call, I am then trying to use it when adding a webinar registrant. I get a 401 “unauthorized” error. I’ll note that the API documentation does not list this as a documented response code? Documented responses are 200, 400, and 404. Zoom Meeting API

I have made sure the App has scopes defined in API documentation (webinar: write: admin, webinar: read: admin, users: view and manage)

The call to add the webinar registrant is using webinar ID in resource path (example: base URI & webinars/93388878833/registrants), and passing same values for Host as token call, and Authorization (Bearer {tokenvalue}), with a space character between the string Bearer and the token value, as header values.

I am a little confused by the documentation on this call in Server-to-Server OAuth
It says “Use OAuth 2.0 to make API calls to Zoom endpoints” but I don’t understand if this means additional steps beyond the example call that is documented (which isn’t really Oauth2?)

How can I amend my call above to successfully add a webinar registrant?