Using XmlDocument as parameter in web api

I have seen people hint at using XmlDocument parameter in web api method but can't work out how it should work.

This works

I have done a POST using the following

POST https://service/api/ HTTP/1.1
Content-Type: application/xml; charset=utf-8
apikey: 969db811-41a5-a9b1-4551-9f7c4ca10a99
Host: abcde
Content-Length: 1651
Expect: 100-continue

<itinerary> 
    <batch> .... etc

to the method code

public HttpResponseMessage Post(HttpRequestMessage value)
{
   var doc = new XmlDocument();
   doc.Load(value.Content.ReadAsStreamAsync().Result);

How would I do this?

I am curious as to what the format of the POST would be and the code for the method if I had a method signature as follows. Is it a good approach to accept XML and\or what are the pros\cons when compared to the previous approach I have shown.

public HttpResponseMessage Post(XmlDocument value)
{

I am enjoying learning about RESTful services so any advice from more experienced people would be appreciated