Necesito enviar HTTP datos XML a una URL que tenga Textarea con el nombre de XMLdata. Mis datos XMl están listos y están dentro de XDocument Sendingxml = xml; pero el código postal que he intentado no está funcionando. Sobre todo porque no sé cómo colocar los datos XML en la variable postData ya que solo acepta cadenas. Este es mi código:

        XDocument Sendingxml = xml;

        //  string Sendingxml = "<?xml version=1.0 encoding=UTF-8> <PluginData> <Credential UserName=avosfieldagent01 AuthenticationToken=61cc3957744742dca238c4dd7cbca702 /><Session><PropertyAddress>5 Crosskey</PropertyAddress><PropertyAddress2/><PropertyCity>California</PropertyCity><PropertyState>CA</PropertyState><PropertyZip>92620</PropertyZip><PropertyType>Condo</PropertyType><SourceReferenceId>45643</SourceReferenceId><SessionId>2013070100158346</SessionId><SessionCompleteReturnURL/><CustomerId/><BluebookOrderCheckSum>681a598cf23f412095f6092c281823e6</BluebookOrderCheckSum><BluebookOrderId>11160</BluebookOrderId> </Session></PluginData>";

        // Create a request using a URL that can receive a post. 
        WebRequest request =
            WebRequest.Create("https://test.bluebookcva.net/order/testdirectplugin/3");
        // Set the Method property of the request to POST.
        request.Method = "POST";

        // Create POST data and convert it to a byte array.
        string postData = "XMLData=" + Sendingxml;    //My problem is here as I need postData  as XDocument.


        byte[] byteArray = Encoding.UTF8.GetBytes(postData);
        // Set the ContentType property of the WebRequest.
        request.ContentType = "application/x-www-form-urlencoded";
        // Set the ContentLength property of the WebRequest.
        request.ContentLength = byteArray.Length;
        // Get the request stream.
        Stream dataStream = request.GetRequestStream();
        // Write the data to the request stream.
        dataStream.Write(byteArray, 0, byteArray.Length);
        // Close the Stream object.
        dataStream.Close();
        // Get the response.
        WebResponse response = request.GetResponse();
        // Display the status.
        Console.WriteLine(((HttpWebResponse)response).StatusDescription);
        // Get the stream containing content returned by the server.
        dataStream = response.GetResponseStream();
        // Open the stream using a StreamReader for easy access.
        StreamReader reader = new StreamReader(dataStream);
        // Read the content.
        string responseFromServer = reader.ReadToEnd();
        // Display the content.
        Console.WriteLine(responseFromServer);
        // Clean up the streams.
        reader.Close();
        dataStream.Close();
        response.Close();