A product key is usually required to activate a plugin you purchased on nopdeveloper.com.
Generally, if you bought one of our plugin, right after a you placed an order, we send out a License Key email with your products license key.
You always can find your plugin licence key code by log into your NopDeveloper.com account and click on “My account” link on the top menu and then go to "Orders" on the side menu. Find the order were you purchase the plugin and click "Details". Your product license key is on the note section of the order information page (see pic below).
If you already installed and activated the plugin, you can also find the license key on your plugin configuration page on your nopCommerce store backend.