I had tried using slmgr /ipk for activating generic product keys for windows server 2012 on Microsoft support web pages
If you're trying to convert an evaluation to full then the correct syntax is
Dism /online /Set-Edition:ServerStandard /AcceptEula /ProductKey:12345-67890-12345-67890-12345
(assumes a Standard edition product key)
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if the reply is helpful--