Windows 10 under VirtualBox

I have a Laptop that came with Win10 preinstalled. I don't use anything Microsoft unless I absolutely have to, so the Primary OS is now Linux with 10 running under VirtualBox when I need it. Problem is, the license key is generic and requires a digital license, which obviously the laptop has, but what is it? Microsoft as usual wants me to throw money at the problem, but I don't use it much and since I already have a valid license I object just on general principals.
So how can I find the magic numbers to use the license I already paid for as part of the laptop purchase?