It is important to keep in mind that the License key and CodeCanyon purchase code are two different things.
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In order to set the License key for the Filter Everything PRO plugin, simply head to Filters -> Settings -> License and click the “Get your License Key” button, please check the image below.
It will take you to a new page where you have to log in with your Envato credentials (with an account where you have purchased the plugin) and obtain your License key.