VAMT 3.0 - Windows ADK for Windows 8.1 "The specified product key is invalid, or is unsupported by this version of VAMT. An update to support additional products may be available online"
Using VAMT 3.1 here (downloaded from the same link above) and get the same error trying to add the Windows Server 2012 for Windows 10 KMS Host key (Windows Srv 2012 DataCtr/Std KMS for Windows 10). The key works fine on the host itself using slmgr.