Welcome to TurnKey! :) No problem at all.

You have a few choices:

1. Possibly the quickest and easiest is to use the server's IP address (instead of the domain). You will need to ensure that your server has a static IP address though.

2. Set up a local DNS server on your network. You can then set up a custom LAN based domain (e.g. home.lan) and give your servers a subdomain of that. This is possibly the "best" and most robust option, but I suggest probably overkill for VirtualBox VMs running on your local PC.

DNS queries for other domains can be forwarded to your DNS provider of choice. Again, you'll need to ensure that the domain maps to IP address. Either give your servers a static IP address, or use a combined DHCP/DNS such as dnsmasq (don't forget to disable other DHCP if you go that route).

3. If you're the only user and/or you're only doing early stage testing (which sounds likely), then adding an entry to your hosts file is pretty easy. Before asking a nameserver for DNS records, your PC will check it's local hosts file. So you can set any domain to point to any IP address within your hosts file. Google something like "edit hosts file MY_OS" (where "MY_OS" is your PC operating system) for instructions on how to do that.

Assuming that you have already set a (non-existent) domain for these appliances, unless you use your local hosts file (see #3 above), you'll want to reset the domain to a new custom one. To do that you'll need to re-run the firstboot scripts. You can always rerun all of them via the turnkey-init command. Alternatively, you can just run the specific appliance firstboot script. You'll almost always find that as /usr/lib/inithooks/bin/APP_NAME.py. E.g. rerun the phpBB specific firstboot scripts like this:

/usr/lib/inithooks/bin/phpbb.py