I'm using another service provider but this is the code:
clientWeb = new WebClient();
clientWeb.QueryString.Add("login", "my_login_name");
clientWeb.QueryString.Add("password", "my_password");
clientWeb.QueryString.Add("message_type", "GP");
clientWeb.QueryString.Add("sender", SmsSender);
clientWeb.QueryString.Add("recipient", PhoneNumber);
clientWeb.QueryString.Add("message", Msg);
clientWeb.QueryString.Add("order_id", Id);
string baseurl = "https://My_URL_Service";
Stream data = clientWeb.OpenRead(baseurl);
StreamReader reader = new StreamReader(data);
StrError = reader.ReadToEnd();
data.Close();
reader.Close();