Passing token from an httpwebrequest in silverlight toward MVC

I have a silverlight application calling an asp.net mvc 3 website (using as webservices application) by using httpwebrequest. i would like to add basic security : send a token representing a guid of an user.

• how to send the token, must i use cred开发者_如何学编程entials attribute ? (seems adding header is not available in silverlight application)
  • how to retrieve the token (or credentials) in MVC controller ?

thanks by advance