OIDRegistrationRequest - Client metadata invalid error

Hello,
I have AppAuth implemented in an IOS project. Everything works fine.

But I tried to use OIDRegistrationRequest, based on the example provided in https://github.com/openid/AppAuth-iOS/blob/master/Examples/Example-iOS_ObjC/Source/AppAuthExampleViewController.m, and i am getting "Client metadata invalid" error.

The code used is:

OIDRegistrationRequest *request =
        [[OIDRegistrationRequest alloc] initWithConfiguration:configuration
                                                 redirectURIs:@[ redirectURI ]
                                                responseTypes:@["code"]
                                                   grantTypes:nil
                                                  subjectType:nil
                                      tokenEndpointAuthMethod:@""
                                         additionalParameters:nil];

    [OIDAuthorizationService performRegistrationRequest:request
        completion:^(OIDRegistrationResponse *_Nullable regResp, NSError *_Nullable error) {

    }];

I am not sure what tokenEndpointAuthMethod should be, documentation isn't helpful and i tried all endpoints

Start with the OIDRegistrationRequest usage in Examples/Example-iOS_ObjC/Source/AppAuthExampleViewController.m and the initializer shown in the issue. Check how tokenEndpointAuthMethod is defined and what the OAuth provider requires; the provider is not identified, so reproduction may require that missing context. Done means establishing a provider-valid registration request without the “Client metadata invalid” error.