using (WebClient client = new WebClient())
{
    client.Headers.Add("Authorization", "YOUR_ACCESS_KEY");
    byte[] response =
    client.UploadValues("http://rest.sendmode.com/v2/hlr", new NameValueCollection()
    {
        { "mobilenumber", "0870000000" }
    });

    string result = System.Text.Encoding.UTF8.GetString(response);
}