Recent observations on another Win10 installation…
I gave my 86-yr old mom my HP Envy i5 desktop that came with Win8. When I first got it, I did a clean install of Win7 and that is what she is using (and won’t upgrade).
I decided last week that when it came time to sell that pc, that it should have Win10 with it. I switched the wires from the current hard drive to a freshly formatted hard drive and installed Win10, skipping the inputting of the product #.
Once it was installed I went to type in the Win8 product # and couldn’t find the sticker on the case (like Dell does) - found out that the original product key is stored in the UEFI (still referred to as the BIOS).
I tried several product key finders (including Magic Jelly Bean) and I liked ProduKey the best - got the Win8 (and Win10) keys and input the Win8. Win10 didn’t like it at first - so I let it sit a couple of days before I tackled it again.
Did updates, installed MS Office, etc, waited another day and when I was poking around trying to find out how to contact MS support - lo, and behold it was activated.
For what it’s worth, looks like the Win10 (free upgrade?) key is the same and tied to your MS account because when I used ProduKey on two other pc’s I have, the Win10 keys were the same - or maybe it’s “my” key assigned to my MS account (I’ll have to go check my son’s pc).