Measuring conversion

Hello,

I'm doing A/B test by changing the movie playing in my homepage to another movie (shorter one). i want to know if it has affect on registrations.

Basically,I want to check if visitors in my homepage watched the video (play video button) and then requested a demo and got up to the "thank you page".

"Play video" (video popup) button ->  Request a demo button (page with form to fill) -> submit - thank you page.

How can i check if people who watch the shorter video on version B reached more or not to the final "thank you" page?

Thanks,

Ido