KMS keys for Windows can be found here: https://technet.microsoft.com/en-us/library/jj612867.aspx
AVMA keys for Windows can be found here: https://technet.microsoft.com/en-us/library/dn303421.aspx

Running winver.exe shows us I’m tunning the Evaluation version:

Using the Activation option to try and activate with a KMS key throws an error:

If you installed the Evaluation version, you first need to convert this edition to the licensed edition.
So, back to good old dism.exe.

In an elevated prompt type the following line:
dism /online /set-edition:ServerStandard /productkey:WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY /accepteula

The product key that is used here is the KMS key for Windows Server 2016 Standard Edition.

Reboot the server (it will reboot twice!).

Checking winver.exe:

In an elevated prompt type the following:
slmgr /ipk WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY

Again, this is the KMS key for Windows Server 2016 Standard Edition.

After that, activate Windows by typing the following in an elevated prompt:
slmgr /ato

Of course, this will only work if you have a KMS host running on your network and have configured the KMS correctly.

Thanks Arjan Mensch