Apache rediret example for webmail.mydomain please

I am interested in implementing one of the new virtualmin features that redirects from webmail.mydomain.com to https://www.mydomain.com:20000, but for my existing sites. Could someone please provide an example?
I know it will do this for newly created sites, but I dont have a spare domain to try it out on right now.


you dont need a spare domain to see it,
just create test.com or even a subdomain for your existing domain.