Creating an authorization request with AppAuth using the 'plain' PKCE challenge method.

AppAuthExampleViewController.m

    // builds authentication request
    NSString *state = [OIDAuthorizationRequest generateState];
    NSString *codeVerifier = [OIDAuthorizationRequest generateCodeVerifier];
    OIDAuthorizationRequest *request =
        [[OIDAuthorizationRequest alloc] initWithConfiguration:configuration
                                                      clientId:kClientID
                                                        scope:@"openid profile"
                                                   redirectURL:redirectURI
                                                  responseType:OIDResponseTypeCode
                                                         state:state
                                                  codeVerifier:codeVerifier
                                                 codeChallenge:codeVerifier
                                           codeChallengeMethod:@"plain"
                                          additionalParameters:nil];