Some APIs want you to supply the appropriate "Accept" header in the request to get the wanted response type.
For example if an API can return data in XML and JSON and you want the JSON result, you would need to set the HttpWebRequest.Accept property to "application/json".
HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create(requestUri);
httpWebRequest.Method = WebRequestMethods.Http.Get;
httpWebRequest.Accept = "application/json";