First of all, it is necessary to install KB2757817 in order to activate a Server 2012 KMS key on Windows Server 2008 R2.
Second, you need to clarify whether you actually have a Server 2012 KMS key. You will not be able to install a Windows 8 KMS key on a server, period. However, server KMS keys typically can activate client operating systems.
Server 2008 R2 KMS keys will activate Server 2008 R2, Server 2008, Windows 7, and Windows Vista.
Server 2012 KMS keys will activate Server 2012, Server 2008 R2, Server 2008, Windows 8, Windows 7, and Windows Vista.
Windows 8 KMS keys cannot be installed on Windows Server operating systems, nor can they activate servers. They will activate Windows 8, Windows 7, and Windows Vista.