C# ส่ง sms โดยใช้ rest api แต่มี error .........................

System.Net.ServicePointManager.Expect100Continue = False
        System.Net.ServicePointManager.UseNagleAlgorithm = False

        Dim RQ = DirectCast(System.Net.WebRequest.Create("yourhost"), System.Net.HttpWebRequest)
        Dim postData As String = String.Format("from:{0}", "sender", "to:{1}", "66911996652", "text:{2}", "test sms")
        Dim data = System.Text.Encoding.UTF8.GetBytes(postData)

        With RQ
            .Method = "POST"
            .ContentType = "application/json"
            .ContentLength = data.Length
            .Headers.Add("Authorization", "Basic " & Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes("username:password")))
            .AllowWriteStreamBuffering = True
            .KeepAlive = False
            .Credentials = System.Net.CredentialCache.DefaultCredentials
        End With

        Using stream = RQ.GetRequestStream()
            stream.Write(data, 0, data.Length)
        End Using

        Dim response = DirectCast(RQ.GetResponse(), System.Net.HttpWebResponse) --> error บรรทัดนี้ครับ
        Dim responseString = New System.IO.StreamReader(response.GetResponseStream()).ReadToEnd()