Limiting navigation until text entry is filled

Hi all,

I have a screen for a practice simulation I'm building in Storyline2 in which the user will fill in their associate and store numbers in two separate fields before proceeding (or that's the idea, anyway). It doesn't matter so much what they enter (since it's practice) but that they fill in both fields before clicking the LogIn button to continue to the next screen.

I know how to set it so all buttons on a slide must be clicked before continuing but this one has me stumped.

Any suggestions?

Thanks!