Getting an XML sent by Webservice(POST method) in C# .NET

I am developing this application where I need to send an XML file to a webservice and read it from the webservice, and applying the business logic from the webservice, it will send an XML as response. The thing is that I know how to read the response the webservice sends, but I'm getting confused as how to read the XML that the request sends to the webservice. If someone could help me out, I'd be really glad.. the XML always comes null(of course, I'm doing something wrong).

Request Method:

(...)
    using (var client = new HttpClient())
        {

            var XMLRequest = Util.BuildXML.CreateRequestXML();

            string url = string.Format(WEB_API_HOST + "/Application/MyMethod/");

            HttpRequestMessage httpRequest = new HttpRequestMessage()
            {
                RequestUri = new Uri(url, UriKind.Absolute),
                Method = HttpMethod.Post,
                Content = new StringContent(XMLRequest.ToString(), Encoding.UTF8, "text/xml")
            };

            var task = client.SendAsync(httpRequest).ContinueWith((taskwithmsg) =>
            {
                var response = taskwithmsg.Result;

                if (response.IsSuccessStatusCode)
                {
                    var xmlResponse = response.Content.ReadAsStringAsync().Result;
                    XmlDocument doc = new XmlDocument();
                    doc.LoadXml(xmlResponse);

                    var test = doc.SelectSingleNode("RESPONSE/...").InnerText;

                }
                else
                {
                    var contentTask = response.Content.ReadAsStringAsync().Result;

                    throw new Exception(contentTask);
                }
            });

            task.Wait();
        }
    (...)

And the Webservice ( that is supposed to receive the XML and read it)

[AcceptVerbs("GET", "POST")]
    public HttpResponseMessage MyMethod(XmlDocument doc)
    {

        // Any doc.SelectSingleNode() wouldn't work, the doc variable is null.

        var response = new StringBuilder();

        response.Append("<?xml version=\"1.0\" standalone=\"yes\"?>");
        response.Append("<RESPONSE>");
        (...)
        response.Append("</RESPONSE>");

        var xmlResponse = new HttpResponseMessage()
        {
            Content = new StringContent(response.ToString(), Encoding.UTF8, "text/xml")
        };

        return xmlResponse;
    }

So how do I get to read the xml in the "MyMethod" method?

Thanks a bunch!