This extension allows you to secure your form in two different ways; using a password or validating license keys against ecommerce systems.
You can make your form hidden until the security has been passed or the verification can be done using fields added to your form.
The license key validation currently supports Envato marketplaces, Sellwire and Easy Digital Downloads. You can define a product for the license key to be validated for or allow the user to select the product from a dropdown in the form.
This plugin requires the free Ninja Forms WordPress plugin.