I’d say that Submit Form and Set Form element Value are there for this reason. There should be examples on how to use them, though I can’t find 'em. But it shouldn’t take too many tries too understand how to operate them (not true at all: I still haven’t found a way; missing some crucial knowledge).
Anyway, if you get to login, there may be issues with GDI, as in the thread linked. You should post what you find, though.