Add a Purchase key or purchase code

Hello Guys!!
How are you?
I want to upload a Perfex CRM Module to envato and need to have some purchase key or purchase code or something in my module, I need to protect myself if anyone try to sell my module in other places or sell to clients directly.
You know how to make this or you have some tutorial to make a purchase key or purchase code personal of the archive someone upload to envato. I mean other purchase key different that the envato purchase key made.

Thanks a lot.

Kind of confused! Do you want to create something like an “activation code” which your users will use to unlock your product. Or do you want to add some hidden string in your code so that you can find if someone tries to resell it!