Enter the blog.domain.tld and whatever password you want to use; keep all other settings at their default values and it should work the way you want - Virtualmin should be able to handle the internal and external IPs automatically.
You have pointed the DNS of your domain to the NS of Virtualmin, right?
I have VPS (CentOS7) and Virtualmin installed years ago. Recently I have sold primary domain, and now I want to use another domain as primary (existing domain on my server). How to do that? Change hostname or there are some other configuration to do? I’m asking, before try anything because domain I want to use is working site and I don’t want to break something.
In short: oldhostname (sold), and now I would like to have mydomain.com to be primary.