It depends on the model. Normally there is a sticker on be device additional the license can be added by the manufacturer in the BIOS. On top of that Microsoft often save the license which is bound to a hardware in there online database. So if you install Windows 10 for example and has an working internet connection the info's can be fetched automatically from that database during the windows 10 installation.
The key which is build into the image is a so called "generic keys" and can only be successfully be used by the manufacturer. You can google for "windows 8.1 generic keys" to find them. You can enter that key to bypass the first dialog but later one need to perform an online activation (if the key fits the device and OS).
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P.S.
I am a volunteer and I do not work for, nor represent, HP
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