All access to the User Center API requires client authentication. A client can authenticate itself with the Client Credentials Grant described in RFC 6749. In short, the client exchanges its client ID and client secret for an access token.
Request
In order to get a client access token, send a GET request to /oauth/token?grant_type=client_credentials. The request should contain a basic HTTP access authorization header with the client ID as a username and the client secret as a password.