Greetings, just installed Z16Pro from an ISO on USB stick that was installed for my own license. I now purchased a Z16Pro license for a client and installed from my ISO, but struggle to find the spot in the settings menu where I can key in the client's new registration number.
Would someone smarter kindly point me in the right direction?
Thank You
You should install from the Clients downloaded copy. If you need, I am sure you can contact them and have them relay the download copy to you for you to install on their system. 
Thank you, are you sure there needs to be a separate ISO downloaded and installed for each license? Or this just as a recommendation?
I'd rather prep a few laptops ready to go when I have spare time, and then key in the client's license once sold.
But new to this OS, so thanks for any help. R.
There is no "license" or registration number. However, your client may need support directly from ZorinGroup at a later date.
So, this is a recommendation as they would need to show their Zorin OS Pro receipt or use their purchase Download Email to contact support.
As far as Distributing goes, I would recommend you use your Zorin Pro Contact Forum to ask the developers @AZorin and @zorink directly to ensure legal compliance.
If you pay for a Z19Pro license, as we do, you are actually sent a unique registration number. We purchase the license under the client's name and email address. It was my assumptions this needs to be associated with the installed copy of the OS on the client's device, just as we do for M$ installs.
Thank you for pointing me to that other forum.
This is for ensuring legitimate copies for the ZorinGroup. This does not need to be entered into the Installed OS, but is instead used for association with the ZorinGroup.
There doesn't seem to be a forum "Zorin Pro Contact Forum"
In the Email that includes the Zorin OS 16 Pro download, you can find the link. Or, you can use this one here:
You can also reply to the email received when made a purchase.
Thank you. Yes, familiar with the support by email. I just thought I may get a quicker pointer here to overcome my partial blindness 