C# api ile sms gönderimi

REQUEST { "username":"user",
"password":"test",
"sdate":"20140214123000",
"vperiod":"48",
"message":{ "sender":"ALFANUMERIK",
"text":"Mesajmetni",
"utf8":"0|1",
"gsm":[ "905445554433", "905445554434" ] } }


http://www.goldmesaj.com.tr/api/v1/sendsms

firma tarafından verilen yukarıdaki api bilgilerini aşağıdaki gibi mesaj yollamaya çalışıyorum aşağıdaki gibi hata alıyorum.

{"status": "error","error": "json formatı hatalı !? hata : Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.\r\n   at Newtonsoft.Json.JsonTextReader.ParseValue()\r\n  at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)\r\n  at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)\r\n   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)\r\n   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)\r\n   at ajaxData.sendSms_Single()","result": "0","sonuc":false,"mesaj":"json formatı hatalı !? hata : Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.\r\n   at Newtonsoft.Json.JsonTextReader.ParseValue()\r\n   at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)\r\n   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)\r\n   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)\r\n   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)\r\n   at ajaxData.sendSms_Single()"   }

 private void button1_Click(object sender, EventArgs e)
        {
    string ss = "<username>" + t_id.Text + "</username><password>" + t_pass.Text + "</password><sdate></sdate><vperiod></vperiod><sender>ALFANUMERIC</sender><text>" + t_mesaj.Text + "</text><utf8>1</utf8>" +"<gsm>" + t_mustel.Text + "</gsm>";

  label8.Text = XmlPost(" http://www.goldmesaj.com.tr/api/v1/sendsms", ss);}

    private string XmlPost(string PostAddress, string xmlData)
        {
            using (WebClient wUpload = new WebClient())
            {
              //  wUpload.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
                Byte[] bPostArray = Encoding.UTF8.GetBytes(xmlData);
                Byte[] bResponse = wUpload.UploadData(PostAddress, "POST", bPostArray);
                Char[] sReturnChars = Encoding.UTF8.GetChars(bResponse);
                string sWebPage = new string(sReturnChars);
                return sWebPage;
            }
        }