How can I request a short "password" at the beginning of my form to identify the person filling out the form with more certainty?

But what other options are there?
There is another post asking something similar:

So it seem people want to use some simple way to have a password. As you can see in my reply in that other question, it is not foolproof but at least you can try and mitigate some of the problems by checking the deviceid too.

In a specific use case I had, they wanted the password and this worked great as the data gathered was not personal or sensible information.