In this tutorial you will learn how to generate license key whether for your client or your own side.
Your customer will send you Hardware ID after he launch Point of Sale system, He can got it from Settings -> Activate License tab, then you will open Rebranding Tool and select License Generator tab. and add his Hardware ID and generate license key for him, you can assign any period you need lifetime, 365 days and so on.

Then open Rebranding Tool -> License Generator Tab

Same for if you need to generate License Key for yourself!, You can copy your own Hardware ID (As its your admin side), Then you will generate lifetime license for yourself.
Thanks,
Cemrox