It's called Captive Portal. There is no generic solution here.
You *could* try to load an arbitrary web page, which would get you an http redirect.
Then load the redirected web page and emulate it's "post" operation.
( and/or some user name/password, multiple web pages, etc. )
One of the arguments against using Captive Portals in general is exactly that.
Sorry for now really being able to help ;-)