UPDATE: You can do that from the GUI too!
If you installed Windows 8 from a volume licence media, you will probably see the following error message when you try to activate Windows:
Activation Error: Code 0x8007232b
DNS Name does not exist
The solution, just like earlier with Windows Server 2008, is to enter a valid product key, that was not requested during installation.
Unfortunately, this new version of Windows does not provide the Change product key option in the Computer properties window, so you will need an elevated Command Prompt:
slmgr.vbs –ipk "ENTER-YOUR-PRODUCT-KEY-HERE"
That’s all, now you can activate Windows without any error (hopefully).