All, it seems there is a typo in the Client Registration documentation lying at:

http://keycloak.github.io/docs/userguide/keycloak-server/html/client-registration.html#d4e1563

String initialAccessToken = "eyJhbGciOiJSUzI1NiJ9.eyJqdGkiOiJmMjJmNzQyYy04ZjNlLTQ2M....";

ClientRepresentation client = new ClientRepresentation();
client.setClientId(CLIENT_ID);

ClientRegistration reg = ClientRegistration.create().url("http://keycloak/auth/realms/myrealm/clients").build();
reg.auth(Auth.token(initialAccessToken));

client = reg.create(client);

String registrationAccessToken = client.getRegistrationAccessToken();