To activate server core

Cscript slmgr.vbs –ato

Activation in Server Core
There is a VBScript in Vista/Longhorn that can be used to activate from the command line: slmgr.vbs.
 

To activate Server Core you can run:

Cscript slmgr.vbs –ato

You can also verify your connection to the Internet and proxy settings (Netsh winhttp proxy)

To verify if activation was successful you can run:

Cscript slmgr.vbs –did

From the output, get the GUID for the appropriate product key, e.g. retail channel, and then run:

Cscript slmgr.vbs –dli GUID

If you didn’t activate before the grace period expired, you can also use slmgr.vbs to remotely activate. This can be done remotely by including the servername, username and password along with the –ato switch.