Password protected web form?

I want to create a webform that is password protected. We go to multiple events and have people fill out forms and then we enter their name in a drawing. Instead of paper forms, I want to have a link on our website where they can fill it out on their phone or tablet to save us time. I also don’t want people who don’t attend the trade show to be able to sign up. The best solution I can think of is to have the web form or landing page password protected. We can have them sign in with the password at the event for their drawing entry. Does anyone know if that is possible?

Exactly what i am looking too, with a little twist…
We have customers, who buy 12 months membership, and in that period they can sing in several events. But only to their membership end date…