Posting XML to a WebAPI method always null

I am performing a systems integration. The client documentation states

"messages are transmitted with the HTTP CONTENT_TYPE header set to “application/x-www-form-urlencoded”.

<Message>
  <Originator>ACME Solutions</Originator>
  <Payload>
     ...message xml content goes here...
  </Payload>
</Message>

I am using WebApi 2 and the default controller action for post has the definition of

public void Post([FromBody]string value)

Everytime I post this message to my web service the value received is always null

I have tried creating a class with the same definition but that always has the properties as null.

public void Post([FromBody]Message value)

If I change the definition to a more generic one I can extract the Xml Document from the request content but thats not the way I want to go as it makes unit testing difficult.

public void Post(HttpRequestMessage request)

Can anyone suggest where I am going wrong and why the xml message isn't mapping to my class.