You can simply serialize the request object, before subtmit, like this:
var sreq = new SomeSoapRequest();
// ... fill in here ...
var serxml = new System.Xml.Serialization.XmlSerializer(sreq.GetType());
var ms = new MemoryStream();
serxml.Serialize(ms, sreq);
string xml = Encoding.UTF8.GetString(ms.ToArray());
// in xml string you have SOAP request