"On tap" is a trigger, as indicated by the green arrow next to it. "On function" is also a trigger, and Construct 2 only allows one trigger per event(including subevents), which is why you can't use "On tap" in a function. Instead, your "On tap" event should call your window creation function.
In relation to your second post, calling a function will return to the branch point after the function has completed -- I'm not sure what gave you the impression that it won't. If you're having issues, I'd suggest posting a capx so someone can help you out with the problem.