Using Virtualmin without a Domain Name for Installation - Just IP?

Is it possible to use Virtualmin without a domain name, just the IP?

I don’t need a domain name for the installation for DNS purposes for the hosted sites as I set up nameservers for each site at the registrar and use these along with modifiying default nameservers generated when Virtualmin creates a virtual server.

You can certainly use a “dummy hostname” for your server, yes. A Linux installation usually does need a hostname, and if possible an FQDN, but you can use something like “myserver.mynetwork.local” or so, if you don’t need name resolution. You might need to tell the first-run wizard to skip resolvability check.