An authenticated onion service is an onion service that requires you to provide an authentication token (in this case, a private key) before accessing the service.
The private key is not transmitted to the service, and it's only used to decrypt its descriptor locally.
You can get the access credentials from the onion service operator. Reach out to the operator and request access.
Learn more about how to use onion authentication in Tor Browser.
If you want to create an onion service with client authentication, please see the Client Authorization in the Community portal.