You can just put a fake license such as the word lic. This will allow you to save the system package and will work with your version of windows.
You can enter anything for the license, and this will be ignored once the OS install takes place. PM requires an entry for this field, even though your copy of Windows may not.
Thanks for the comments.
I'm also trying to understand if you are using a select license you don't get asked for the code during installation. Is there a way to bypass this e.g. sticking something in the unattend.txt section?