Provide the callback url by the delegate and make sure autoStart is NO

gistfile1.m

  NSDictionary *credentials = [NSDictionary dictionaryWithObjectsAndKeys:	
                               consumerKey, kMPOAuthCredentialConsumerKey,
                               consumerSecret, kMPOAuthCredentialConsumerSecret,
                               nil];
  
  _oauthAPI = [[MPOAuthAPI alloc] initWithCredentials:credentials
                                    authenticationURL:[NSURL URLWithString:@"https://api.login.yahoo.com/oauth/v2/"]
                                           andBaseURL:[NSURL URLWithString:@"https://api.login.yahoo.com/"] 
                                            autoStart:NO];
  
  [(MPOAuthAuthenticationMethodOAuth *)[_oauthAPI authenticationMethod] setDelegate:self];
  [_oauthAPI authenticate];