Yeah, like I said previously, you don't need your Windows product key because...
A. Your Windows 10 product key is not unique. It's just a generic key and meaningless.
B. Your Windows 8.1 key is stored in the UEFI and you don't need to enter it for either Windows 8.1 or Windows 10 installation. It will be automatically retrieved. If you had to (and you won't), you can manually retrieve your Windows 8.1 key from a Windows 8.1 or Windows 10 USB media from a command prompt.
C. Your Windows 8.1 key would only be necessary if you had a retail copy of Windows 8.1. And you don't have a retail copy....you have an OEM copy of Windows and the key is stored in the UEFI. And your Windows 10 license is a digital entitlement (digital license) linked to your Microsoft account.