To add a subdomain to your DNS server, select domain, then go to:Webmin Modules -> BIND DNS ServerClick the zone you want to edit.Click "Name Alias Records"And add the Name Alias Records (CNAME)click Create button to create it.Then click "Apply Zone" top right to update name servers.Similarly, you can change the name-records of your DNS server. But be careful, it could lead to your domain not being reachable anymore.