Then you have found the Product ID which has no particular use. It is derived in part from the Windows version with the variable part (what makes it the individual serial number) decoded from the Activation key.
The "hidden" value is the Activation Key. It is not printed or stored anywhere as a readable value but there are plenty of tools available to extract it from the system.
Microsoft did not envisage any particular scenario which needed the key to be known by the user for an OEM system hence no copy of the key was provided. As the key is embedded into the BIOS Windows setup should pick up the key automatically without user
entry.
Even when with Windows 7 (and before) a label was provided with a key printed this was about having a visible proof of license. It had security features making harder to copy and although it had an Activation Key printed on it the factory Windows installation
did not use that key.