SOAP Request XML with User Credentials

private string SAR(XmlDocument requestXML, string webServiceName)
{
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(webServiceBasePath + webServiceName);
request.Headers.Add("SOAPAction", webServiceBasePath + webServiceName);
request.Credentials = new NetworkCredential(kulAdi, kulSif);
request.ContentType = "text/xml";
request.Method = "POST";
request.Accept = "text/xml";
Stream stm = request.GetRequestStream();
stm.Write(System.Text.Encoding.ASCII.GetBytes(requestXML.InnerXml.ToString()), 0, requestXML.InnerXml.ToString().Length);
stm.Close();
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
StreamReader r = new StreamReader(response.GetResponseStream(), System.Text.Encoding.UTF8);
return r.ReadToEnd();
}