Sometimes we've just built our login routine in the traditional Seam way using the Identity instance wired to a form like:
In order to login the user automatically when i.e. clicking on a link we just need to add the following to pages.xml and to our Authenticator
So now the parameters will be automatically propagated by Seam and we can programmatically call login() on the Identity instance. In this example the URL would be /login.seam?username=xxx&password=xxx
Of course we could use the URL rewriter to make Seam react to more RESTful URLs. See the docs for that.