How can I password protect a form?

You can do that by doing a combination of conditional logic and calculation. Please refer to the guide below for the full set up:

https://www.jotform.com/help/306-Allow-Jotform-to-Be-Filled-out-Only-by-Those-with-a-Special-Link 

If you cannot get it to work, please let us know here and provide us the form, and we will check.