Hi,
If your notebook originally shipped with Windows 8, the activation key is stored in the Bios chip and will be automatically read during the installation process.
This will also work if you create your own OEM Windows 8 installation disc - see the 3rd post by Huffer on the following link.
http://h30434.www3.hp.com/t5/Notebook-Hardware/Need-Product-key/m-p/2592099/highlight/false#M96668
Regards,
DP-K