After you’ve logged into Windows Server Core, you’re presented with the Command Prompt. From there, you can activate your copy of Windows.
At a windows prompt type the following and read the instructions carefully.
slmgr.vbs
This will allow you to obtain an activation code by phone from the license center and do an offline activation.
The license center will help you to set up a batch activation from a file or, at least they used to be helpful with this.
Here's the breakdown of what to do, for anyone reading this later.
slmgr <computer name> /ipk <product key>
slmgr <computer name> /dti
(This will give you a popup box with a long string, which you'll have to submit for activation; I prefer using the mobile website. The string doesn't have spaces or dashes, so be careful to get it right)
slmgr <computer name> /atp <activation id>
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