There are 2 typical licenses for Windows 10 and VDI, if you plan to use Instant/linked Clones then KMS is the only way. If you want to test and don't have KMS you can use the URL i put in the other reply and disable that requirement.
Multiple activation key
A Multiple Activation Key (MAK) is commonly used in small- or mid-sized organizations that have a volume licensing agreement, but they do not meet the requirements to operate a KMS or they prefer a simpler approach. A MAK also allows permanent activation of computers that are isolated from the KMS or are part of an isolated network that does not have enough computers to use the KMS.
Key Management Service
With the Key Management Service (KMS), IT pros can complete activations on their local network, eliminating the need for individual computers to connect to Microsoft for product activation. The KMS is a lightweight service that does not require a dedicated system and can easily be cohosted on a system that provides other services.
Plan for volume activation (Windows 10) - Windows Deployment | Microsoft Docs
If you have a KMS server on your domain you need to use a certain KEY in Windows, this key tells the OS that there is a KMS server assuming thats all setup. Find them at following URL
KMS client setup keys | Microsoft Docs
Hope this helps
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