I ran into this same problem. One way to do this is to create a password protected page and instead of attaching the form to the page, use the form’s shortcode in the page ‘s body. For example:
[ninja_forms_display_form id=1]
The page is password protected and only displays the form after someone enters the correct password.
You can find the shortcode in the second column of the Ninja Forms screen in wp-admin: https://yoursite.com/wp-admin/admin.php?page=ninja-forms
