Firstly before anyone says that the following is not ok to do I would like to point out that it is totally fine to do this and it is documented by Microsoft on TechNet at the following link http://technet.micro...y/hh824952.aspx
You have to use the Product Key from MSDN, otherwise the installer won't accept it. Kind of weird honestly how they set that up because 8.1 can activate with a 8 key.
That isn't totally correct. The ISO images for 8.1 RTM do not include an ei.cfg file in the sources directory however you can add one so that you are not prompted for a key until after installation at which time you can use your Windows 8 product key to activate.
If you want to do this simply use Notepad to create a text file with the following content and save it as ei.cfg in the sources folder on your install media.
If you have a "core" (that is not Windows 8 Pro) license use the following
If you have a Professional license use the following
When you boot up to install Windows 8.1 it will now automatically know the version you are using so will not ask you for a product key before installation however on first boot after the install has completed it will ask you for a key however you are able to click "skip" on this screen and then enter your key when you are ready from the activation wizard.
Hope this helps