You don't need to activate that iso. Just connect to the internet and it will activate automatically...
Server needs KMS activation. OEMRET: Only when there is an established HWID for home/pro or their N version.
It's preview build. It does not need activation. I would at least expect that much from microsoft. Correct me if I am wrong...
Preview builds need (KMS) activation for years now. The time an insider preview account activated any windows is over for a few years now.
I guess that is the reason why it automatically activated after I connected to the internet. Before I had Windows 10 Pro digital license from windows 7 to windows 10 upgrade that is liked to the hardware...