There are many DNS requests that domain joined computers perform in the background (DC search and auth, user sync, AD group policies update, etc). The client computer assumes that at least one domain controller is network-reachable, and without this resource all of those queries will not be sent to the CloudGen Access Proxy, and AD functionalities will begin to fail.
To configure an Active Directory:
Go to Access > Resources.
Add a new resource.
Fill in the required fields.
Add an Active Directory resource (or a wildcard resource).
Go to Devices > Settings.
Scroll down to Agent DNS.
Click the pencil icon on the right.
Click Add New DNS.
Fill in the required fields (see DNS Servers drop-down).
Click Save.