As there is no GUI in Windows Server 2012 Core, we have to use command line and script in order to acivate system.

slmgr.vbs is a VBscript which allows to set product key and activate operating system. Below most common use of slmgr.vbs.

Set product key

slmgr.vbs /ipk XXXXX-XXXXX-XXXXX-XXXXX

Activate Windows

slmgr.vbs /ato

Display information about licenses

slmgr.vbs /dli
slmgr.vbs /dlv