Add licensing code to WordPress Plugin(repost)

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] The license would have a prefix as we have many licensed products and would want to prevent licenses being used on other products. The 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. For example, if the user enters "123456789" as their license key, it would need to submit the license key "premcoupon123456789" to whmcs for authentication. I have attached a sample of the licensing code. Please ask if you have any questions.