Trouble with MultiPart/Form-Data as a web request

I have a HTML POST request which i need to replicate in c#

The HTML is something like

<FORM action="http://RemoteServerURL" enctype="multipart/form-data" method=POST>
    <TEXTAREA id="TextAreaXML" name="xmlmsg" rows="20" cols="100">   </TEXTAREA>
    <button type="submit">Send</button>
</form>

The TextArea Expects an inout which is as below

<?xml version="1.0" encoding="utf-8"?>
<OnlineCheck>
  <Header>
    <BuyerAccountId>XXXXXX</BuyerAccountId>
    <AuthCode>XXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX</AuthCode>
    <Type>STOCK</Type>
  </Header>
  <Item line="1">
    <ManufacturerItemIdentifier />
    <DistributorItemIdentifier>3109750</DistributorItemIdentifier>
    <Quantity>7</Quantity>
  </Item>
</OnlineCheck>

This Part works Fine.

Now i tried to replicate it in c# like below.

WebRequest req = WebRequest.Create("http://RemoteServerURL");
            string xmlmsg = "<?xml version=" + '"' + "1.0" + '"' + " encoding==" + '"' + "utf-8" + '"' + "?><OnlineCheck><Header><BuyerAccountId>XXXXXX</BuyerAccountId><AuthCode>XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXX</AuthCode><Type>STOCK</Type></Header><Item line=" + '"' + "1" + '"' + "><ManufacturerItemIdentifier /><DistributorItemIdentifier>3109750</DistributorItemIdentifier><Quantity>7</Quantity></Item></OnlineCheck>";

            byte[] send = Encoding.Default.GetBytes(xmlmsg);
            req.Method = "POST";
            req.ContentType = "multipart/form-data";
            req.ContentLength = send.Length;

            Stream sout = req.GetRequestStream();
            sout.Write(send, 0, send.Length);
            sout.Flush();
            sout.Close();

            WebResponse res = req.GetResponse();
            StreamReader sr = new StreamReader(res.GetResponseStream());
            string returnvalue = sr.ReadToEnd();

This fails and I get the message

"The request was rejected because no multipart boundary was found XML request"

. So i know the request to the server is working. But the input or something is going wrong. Please Help