ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

CookieCollection cookies = new CookieCollection();
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("my provider logon page here");
request.CookieContainer = new CookieContainer();
request.CookieContainer.Add(cookies);
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
cookies = response.Cookies;

string getUrl = "my provider logon page here";
string postData = String.Format("UserName={0}&Password={1}&userType={2}&rememberMe={3}", "login", "password", "FON", "true");
HttpWebRequest getRequest = (HttpWebRequest)WebRequest.Create(getUrl);
getRequest.CookieContainer = new CookieContainer();
getRequest.CookieContainer.Add(cookies);
getRequest.Method = WebRequestMethods.Http.Post;
getRequest.UserAgent = "Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.110 Safari/537.36";
getRequest.AllowWriteStreamBuffering = true;
getRequest.ProtocolVersion = HttpVersion.Version11;
getRequest.AllowAutoRedirect = true;
getRequest.ContentType = "application/x-www-form-urlencoded";

byte[] byteArray = Encoding.ASCII.GetBytes(postData);
getRequest.ContentLength = byteArray.Length;
Stream newStream = getRequest.GetRequestStream();
newStream.Write(byteArray, 0, byteArray.Length);
newStream.Close();
HttpWebResponse getResponse = (HttpWebResponse)getRequest.GetResponse();
using (StreamReader sr = new StreamReader(getResponse.GetResponseStream()))
{
   string sourceCode = sr.ReadToEnd();
}