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
Display information about licenses
slmgr.vbs /dli slmgr.vbs /dlv