Where is the DNS hosted for this particular domain?
If it’s being hosted on your Virtualmin Server, the first thing you’d need to do is go into Server Configuration -> DNS Records, look for the record named “m”, and set that to point to the IP address of where the site will be hosted.
If DNS is hosted at your domain registrar, then you’d add a DNS record for m.ThisDomain.com at your registrar.
But yeah, all you have to do is add a DNS record where ever the DNS is being hosted, then you can setup your mobile site.