You’re not making any sense. What hostname are you talking about?
If you setup a domain named
example.tld in Virtualmin, and get a Let’s Encrypt certificate for it, it will work for Webmin. Any aliases that you get a certificate for (Virtualmin has some automatic aliases, but you can also create your own, and assuming DNS is correct, they will also work and can also request certs from LE) will also work for Webmin.
Ports are not specified in certificates. Any service on any port can use a certificate. A cert is for a name, not a port or service.
Be careful doing that. People on the internet very often manage to make simple things wildly complicated and break the simple paths in weird ways. I strongly recommend you follow our docs, and use the built-in features, exclusively, until you really know what you’re doing. I haven’t read the docs you linked (and won’t, because I’m sure they’re wrong and overly complicated and that’ll just make me mad).
You don’t need to follow any docs for this. If you have a certificate for any Virtualmin managed domain, you’re done. Stop poking at it, stop making things hard. Just use the Virtualmin managed name to connect to Webmin.