The Jenkins 1 remedy for this issue is to peg the submit button at the bottom of the visible window. This is a pretty reasonable and standard solution, I don't see any reason to stop doing that, especially when the following configuration page would share the same behavior.
Also, in Jenkins 1, the submit button has a visible change of state from inactive to active as the 2 form requirements are met. This is also a reasonable and clear solution for clarifying when the form criteria need to be met.
If this still seems deficient (users might not know why the button is still inactive) we could add a nag to the button when it is clicked in an inactive state, explaining what tasks remain for the user. This added nag may or may not be necessary.