The goal is to setup the VM to be able to access the internet, but also have a separate network connecting the Host and Guest machines.
Adding the Host-Only adapter
Under the Virtualmin GUI Console, select File > Preferences. Next, select the Network item to pull up the list of Network adapters. Create a new Host-Only network adapter. Note the IP address created. The default is 192.168.56.x, but you can change it to whatever you want.