Если в параметрах проекта указан тип клиента, вам просто нужно инициализировать HttpClient (), не указав его тип.
Из документов:
// This will use the default message handler for the application; as
// set in the Project Options for the project.
HttpClient client = new HttpClient();
// This will create an HttpClient that explicitly uses the CFNetworkHandler
HttpClient client = new HttpClient(new CFNetworkHandler());
// This will create an HttpClient that explicitly uses NSUrlSessionHandler
HttpClient client = new HttpClient(new NSUrlSessionHandler());