By using XDeclaration. This will add the declaration.
But with ToString()
you will not get the desired output.
You need to use XDocument.Save()
with one of his methods.
Full sample:
var doc = new XDocument(
new XDeclaration("1.0", "utf-16", "yes"),
new XElement("blah", "blih"));
var wr = new StringWriter();
doc.Save(wr);
Console.Write(wr.ToString());