When deploying an Windows image in MDT 2013 you can add rules in CustomSettings.ini to offer a product key during deployment (for example). Settings in this ini file are used to create the unattend file, which is needed during mini-setup. Just add the following rules for skipping the product key:
When using multiple Windows versions however, this will not do the job. In that case add the product key in the deployment task sequence(s). Just add it at start in the task sequence, beneath the Initialization phase. Just name the variable ProductKey and enter a KMS Client Setup Key (for example). It's just that simple ;)
When looking for KMS Client Setup Keys have a look here: http://technet.microsoft.com/en-us/library/jj612867.aspx
Just love MDT because of simplicity and functionality!