Azure AD Domain Services (AADDS) is ONLY available for classic virtual networks (VNet guidelines), therefore, you might think that it is only possible to domain join classic VMs to a domain.
Not exactly true, it is also POSSIBLE to join an Azure Resource Manager (ARM) virtual machine. However, you first need to:
- Enable AADDS on a classic VNET
- Create a VPN Site-to-Site between the classic VNet and an ARM VNET (view article)
- Connect the VM to the ARM VNET
- Now you can domain join the ARM VM using AADDS
TIP: you might want to change the DNS servers for the ARM VNet to, instead of using Azure DNS, use the DNS provided by AADDS service, which you can get from Azure Active Directory -> Select your directory -> Configure -> Domain Services.