Whenever you setup Virtualmin, it configures your system to serve websites out of /home.
What you would want to do is log into Virtualmin, and choose a domain you want to act as the default domain.
Then, go into Server Configuration -> Website Options, and set it to be the default domain for your IP address.
After doing that, it’ll be the domain that’s served when someone browsed to your domain using the IP.
All you need to do from there is make sure that you’ve uploaded a website into that domain – getting a “Forbidden” message usually means no website has been uploaded yet.