In Windows Server 2012, it asks for a product key as part of the installation process as shown in the following screenshot.
The only way to progress this screen is to enter a MAK key obtained of a companies Microsoft licensing portal.
So what happens if you want the Windows 2012 Server or Windows 8 machine to automatically license itself against a KMS upon completion of the installation? You can't enter a product key here as this means the machine will become a MAK license.
The answer is to use a KMS Client Setup Key. This isn't the KMS key itself, but a key which tells the Windows Server 2012 or Windows 8 machine that once it finishes installation to go off and license itself against a KMS.
Microsoft has published a list of KMS Client Setup Keys on the following TechNet article. Simply enter the correct KMS Client Setup Key to complete the installation in which Windows will go off and automatically activate against the appropriate KMS server through DNS or Active Directory discovery.
Hope this post has been helpful!