You can use WebClient.
Or (if you need more fine-grained control over the request) HttpWebRequest
Or, HttpClient in System.Net.Http.dll.
Here's a "translation" to HttpWebRequest (needed rather than WebClient in order to set the referrer). (Uses System.Net and System.IO):
HttpWebRequest http = (HttpWebRequest)HttpWebRequest.Create(requestUrl))
http.Referer = referrer;
HttpWebResponse response = (HttpWebResponse )http.GetResponse();
using (StreamReader sr = new StreamReader(response.GetResponseStream()))
{
string responseJson = sr.ReadToEnd();
// more stuff
}