Web API accepting XML and Calling the API

I want to create a API that takes XML as argument. I want to host that in a sub domain called abc.xyz.com and give that to my sub company.

I have created the WEB API using MVC-4. A controller called NewRequest that have a post method like this

[HttpPost]
        public string Post(System.Net.Http.HttpRequestMessage request)
        {
           XmlDocument doc = new XmlDocument();
           doc.Load(request.Content.ReadAsStreamAsync().Result);
           string xmlstring=  doc.InnerText.ToString();
           // Then a lot of Logic and finally return the string
          return xmlstring;

        }

then i have hosted the API and i tried to call the API from a web page. But on doing so i am getting an 500 error.

private void send(XmlDocument doc, string url)
{
 string xml = doc.OuterXml.ToString();
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
                req.KeepAlive = false;
                req.ProtocolVersion = HttpVersion.Version10;
                req.ServicePoint.ConnectionLimit = 1;               
                byte[] requestBytes = System.Text.Encoding.ASCII.GetBytes(xml);
                req.Method = "POST";
                req.ContentType = "application/xml";
                Stream requestStream = req.GetRequestStream();
                requestStream.Write(requestBytes, 0, requestBytes.Length);
                requestStream.Close();
                WebResponse res = (WebResponse)req.GetResponse();
                Stream responseStream = (res.GetResponseStream());

}

Can anybody tell me what will be the issue, your support is highly appreciated.

Thanks

RK