Yeah, I see Windows 10 ISO's and associated client keys on my MLVS site now too, but no 2012 R2 w/ Win 10 KMS key. Keep this thread updated it you find the answer.

Learn how to find the type of your Windows 11/10 License or Product Key. Know if it's Retail, OEM or Volume License (MAK/KMS) key.

You need a 2012r2 KMS key to activate both hosts and clients at 8.1 level, do you have a valid SA on your 2012 license? Otherwise you will have to use a different host for KMS duties.