Not really any way reliably to do it. Though you could use a server-side script to process a login form of their user/pass that then adds the user/pass into a URL redirect to the proper location (http://user
***@url). Though this is extremely insecure, leaves the user/pass mix in the history/cache for misuse and various other bad things...it basically negates authentication (though using a hidden function to simulate a login form does negate some
of the bad vibes all around, but not enough to be called 'secure').
In theory you could also hadd the authentication into headers with a similar method, but I've never tried it so I'm not saying 100% one way or the other