WebRequest myHttpWebRequest = WebRequest.Create("http://abc.com/xxx.aspx");
// Set the 'Method' property of the 'Webrequest' to 'POST'.
myHttpWebRequest.Method = "POST";

// Create a new string object to POST data to the Url.
string postData = @"<?xml version="1.0" encoding="UTF-8" ?>
<ROOT>
  <CONFIG>
    <TYPE>IN </TYPE>
    <WORKTYPE>2 </WORKTYPE>
  </CONFIG>
  <DATA>
    <POLICY>
      <UNITCODE>分公司代码 </UNITCODE>
      <APPLYNO>投保单号码 </APPLYNO>
      <APPLYENDORSENO>批单申请号码 </APPLYENDORSENO>
    </POLICY>
  </DATA>
</ROOT> ";

ASCIIEncoding encoding = new ASCIIEncoding ();
byte[] byte1 = encoding.GetBytes (postData);

// Set the content type of the data being posted.
myHttpWebRequest.ContentType = "application/x-www-form-urlencoded";

// Set the content length of the string being posted.
myHttpWebRequest.ContentLength = byte1.Length;

Stream newStream = myHttpWebRequest.GetRequestStream ();

newStream.Write (byte1, 0, byte1.Length);
// Close the Stream object.
newStream.Close ();

HttpWebResponse response = myHttpWebRequest.GetResponse();

StreamReader reader = new StreamReader (Reqeust.InputStream);
String xml = reader.ReadToEnd();