Hello @Daniel_Server and welcome to the community.
The first thing that you must do is configure the DNS records of your domain, via the NameCheap control panel, in the manner described below:
After that, rerun the Post-install Wizard via Virtualmin → System Settings → Re-Run Installation Wizard and be sure to correctly apply the correct domain at the various stages of the wizard, particularly for nameservers.
Finally, you need to apply for a SSL certificate for your domain via Virtualmin → Server Configuration → SSL certificate.
Your domain should then show the lock icon, else give us a holler here with info about the steps that you took and the output of each step.