New PC shipped with windows OEM version from vendor no loner comes with Certificate of Authenticity attached, instead, PC manufacture embedded the windows key into BIOS or EFI.
Please read the link below carefully to understand your downgrade rights, https://www.microsoft.com/OEM/en/licensing/sblicensing/Pages/downgrade_rights.aspx#fbid=OtIyotJ5rw5
So, I just ordered a new PC that comes with windows 10 but due to some application compatibility, the software only support windows 8.1. So i have to downgrade and format the PC to windows 8.1.
I need a product key but it is embedded with the BIOS. I downloaded this tool to get they key, https://neosmart.net/OemKey/ . This product key can be used to re-install window 10 or downgrade to 8.1 with a clean windows setup CD.