Add licensing code to WordPress Plugin

We have obtained a new business that sells a plugin. We need licensing code added to the plugin so that we can control the use of it. We use WHMCS licensing system, you can read more about this here: [login to view URL] There are two versions of the plugin and both will have separate prefixes to the license key. As such, only the free license key should work for the free plugin and only the premium license should work for the premium plugin. The free license prefix would be freecoupon while the premium license prefix would be premcoupon. When the license is checked against our system, it should add the prefix to the beginning of the license key that the user entered. I have attached a sample of the licensing code. Please ask if you have any questions.