Explanation:
Box 1: Configure virtual network peering between Vnet1 and Vnet2.
Connectivity issues for domain-join.
If the VM can't find the managed domain, there's usually a network connection or configuration issue. Review the following troubleshooting steps to locate and resolve the issue:
1. Ensure the VM is connected to the same, or a peered, virtual network as the managed domain.
If not, the VM can't find and connect to the domain in order to join.
If the VM isn't connected to the same virtual network, confirm that the virtual networking peering or VPN connection is Active or Connected to allow the traffic to flow correctly.
2. Try to ping the domain using the domain name of the managed domain, such as ping aaddscontoso.com.
* If the ping response fails, try to ping the IP addresses for the domain displayed on the overview page in the portal for your managed domain, such as ping 10.0.0.4.
* If you can successfully ping the IP address but not the domain, DNS may be incorrectly configured. Make sure that you've configured the managed domain DNS servers for the virtual network.
Box 2: Add a virtual network link to an existing Azure private DNS zone.
The private DNS zone already exists.
After you create a private DNS zone in Azure, you'll need to link a virtual network to it. Once linked, VMs hosted in that virtual network can access the private DNS zone. Every private DNS zone has a collection of virtual network link child resources. Each one of these resources represents a connection to a virtual network. A virtual network can be linked to private DNS zone as a registration or as a resolution virtual network.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory-domain-services/troubleshoot-domain-join
https://docs.microsoft.com/en-us/azure/dns/private-dns-virtual-network-links