What do you do if you want to activate the 2 (or more) Hyper-V VMs that come with your respected licensing? You don’t actually need to use the original Server key. Instead, use these keys:

Windows Server 2016 Edition	AVMA key
Datacenter	TMJ3Y-NTRTM-FJYXT-T22BY-CWG3J
Standard	C3RCX-M6NRP-6CXC9-TW2F2-4RHYD
Essentials	B4YNW-62DX9-W8V6M-82649-MHBKQ

Windows Server 2012 R2 Edition	AVMA key
Datacenter	Y4TGP-NPTV9-HTC2H-7MGQ3-DV4TW
Standard	DBGBW-NPF86-BJVTX-K3WKJ-MTB6V
Essentials	K2XGM-NMBT3-2R6Q8-WF2FK-P36R2

In order for this to work you need to have the Data Exchange integration enabled on the VM via Hyper-V Management.

If you want to enter a Product Key using PowerShell use the following command:

slmgr –ipk

for example: slmgr –ipk TMJ3Y-NTRTM-FJYXT-T22BY-CWG3J