I’ts possible to convert your Server 2016 evaluation version to a production version using one of the following commands depending on your version:
DISM /online /Set-Edition:Serverstandard /ProductKey:WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY /AcceptEula /Norestart
DISM /online /Set-Edition:ServerDatacenter /ProductKey:CB7KF-BWN84-R7R2Y-793K2-8XDDG /AcceptEula /Norestart
Source: KMS Client Keys
Make sure you’ve installed all windows updates first before continuing. Keep in mind that it can take a while to complete. In my case it was stuck at 10% for a long time. Just let it complete and reboot afterwards.