As introduced with Windows 7, the operating system is being installed without the need to provide a product key. Actually this is a good thing, because during the setup you needed to type the whole key, no copy/paste.
In Windows 8 it’s again a bit different. After opening the computer properties in order to activate the system, there is no more button Change License Key. All you have is a message stating that Windows is not activated.
To change the product key, you have two options:
Open an elevated command prompt and type: slmgr -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
Open an elevated command prompt and type: slui 3
After this a similar window as in Windows 7 appears.
Now just paste the product key and you’re good to go!