My hostname for the Virtualmin server is vps01.indiax.com and the name servers I use are dns1.indiax.com and dns2.indiax.com. I use my domain registrar’s control panel to point vps01, dns1 and dns2 to the ip address of the server on which I have installed Virtualmin.
In Virtualmin on vps01.indiax.com I create a virtual server named vps01.indiax.com (but disable mail) and use it as the default website for the server. I apply for SSL certificates using Let’s Encrypt and then use the ‘Copy to Webmin’ and associated buttons to apply the SSL certificate of vps01 to Postfix, Dovecot etc.
One little trick that you could use @Centaro is to configure your domain registrar’s control panel to point the ns record of your domain to the name servers you have in virtulmin. This will let you create subdomains under your hostname as virtual servers in Virtulmin without needing you to manually update your domain registrars control panel. Dig vps07.indiax.com and to see how this works.