How to add a license server into the AD group "terminal server license servers

We have some W2K12 terminal server running in Azure. As ADS we use the Azure AD. There exists no on-premise domain.

Now I want to add a license server into the AD group "terminal server license servers", but the "Add" button in greyed out:

How I can reach my goal to add the license server inside this group?

By the way, I'm 'only' a member of "AAD DC Administrators" and domain or enterprise admin.

Many greetings,

Thorsten