I need a password required protected page submit button

I have a website that advertises potential investors to my clients product. In order to view a special "Information Page" I have generated a standard form for potential investors to fill out before they can proceed to the next page that includes the privy information.

After they have filled out the form I want a way for them to go back to that information page without having to fill out the same form again, and without having to remember the URL address.

I need an initial "Password required" submit button that either requires a specific predetermined set of characters of my choice, and/or simply to enter any proper number of digits in the box next to it's submit button, so that returning visitors, that have been emailed the "secret" code can simply enter it, and hit it's submit "Enter" button, instead of having to re-fill out the whole form again.