Domain redirection
If you just want the new domain to forward (redirect) to your existing domain, log in to your Virtualmin panel and click "Create Virtual Server" and select "Alias of yourdomain.com".
Separate website
Log in to your Virtualmin panel and click "Create Virtual Server" and select "Top-level server".
In any case
Don't forget to point your domain to the right nameservers. You can check our article about that in this knowledge base.