I looked into this a couple years ago. To do legitimately on-prem on a server (vs a user running it in a VM on their workstation), there was a per year user volume or o365 per user monthly SKU that allowed specifically this. For those saying "well just activate pro", yes it will activate but it's not legit. Activation does not equal compliance (it says that in the W10 pro license agreement). I no longer have the SKU or name (Windows VDA iirc?), but here's a link to the PDF:
You likely want to look on page 3:
"You can run Windows 10 virtual machines on your own on-premises server or on a server dedicated to you that is managed by an outsourcer. However, if the outsourcer is not deemed an Authorized Outsourcer (see definition below), only Windows VDA E3/E5 licenses confer virtualization entitlements."
Don't hold me to the details, but i feel like it was $25 per user per month? D&H's license desk helped me track it down, but we ended up not using it.
Did some email searching, i believe it was called "Windows 10 Enterprise E3 VDA" a couple years ago.