Code on test page:
        public ActionResult OnGage()
        {
            Utilities.API.OnGage.Add_Contact("List_id", "Email", "Firstname", "Lastname", "Gender"); 
            return Content("OK");
        }

OnGage API Class:
        public static void Add_Contact(string list_id, string email, string firstname, string lastname, string gender)
        {
            string json = "{ \"email\": \"" + email + "\", \"list_id\": \"" + list_id + "\", \"firstname\": \"" + firstname + "\", \"lastname\": \"" + lastname + "\", \"gender\": \"" + gender + "\" }";
            GenericWebRequest.Send_POST_WebRequest(_url + "/v2/contacts", json, "application/json", "USERNAME", “PASSWORD“, "ACCOUNT CODE");
        }

 GenericWebRequest Class:
public static string Send_POST_WebRequest(string url, string POST_parameters, string contentType = "", string username = "", string password = "", string account_code = "")
        {
            string webResponce = string.Empty;

            try
            {
                HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
                webRequest.Method = "POST";
                byte[] dataStream = Encoding.UTF8.GetBytes(POST_parameters);
                webRequest.ContentLength = dataStream.Length;
                if(contentType == "") webRequest.ContentType = "application/x-www-form-urlencoded";
                else webRequest.ContentType = contentType;
                //if password protected
                if (username != "" && password != "")
                {
                    if (account_code == "") webRequest.Credentials = new NetworkCredential(username, password);
                    else
                    {
                        webRequest.Headers.Add("X_USERNAME", username);
                        webRequest.Headers.Add("X_PASSWORD", password);
                        webRequest.Headers.Add("X_ACCOUNT_CODE", account_code);
                    }
                }

                // Send the data.
                Stream newStream = webRequest.GetRequestStream();
                newStream.Write(dataStream, 0, dataStream.Length);
                newStream.Close();
                WebResponse webResponse = webRequest.GetResponse();

                using (StreamReader rd = new StreamReader(webResponse.GetResponseStream()))
                {
                    webResponce = rd.ReadToEnd();
                }

                return webResponce;
            }
            catch (WebException ex)
            {
                try
                {
                    webResponce = new StreamReader(ex.Response.GetResponseStream()).ReadToEnd();
                    Log.Logger.LogError("WebException while trying to do generic web (POST) request with the request url: " + url + " webresponce: " + webResponce, "Send_WebRequest", "GenericWebRequest", true);
                }
                catch (Exception e)
                {
                    Log.Logger.LogError("Exception while trying to do generic web (POST) request with the request url: " + url, new System.Diagnostics.StackFrame(), e, true);
                }
                return webResponce;
            }
            catch (Exception ex)
            {
                Utilities.Log.Logger.LogError("Error while trying to do generic web (POST) request with the request url: " + url, new System.Diagnostics.StackFrame(), ex, true);
                return webResponce;
            }
        }