I followed this document: https://www.virtualmin.com/documentation/web/faq
Now when I goto webmail.domain.org it forwards to port 10000 to virtualmin and that could't be further than what I intended. webmail.domain.org has never forwarded to the domain I put in either. I'm not really sure what I can be doing wrong, the instructions seem straightforward. Thank you.
EDIT: It seems there's a specific order you must do this in. I enabled the webmail forwarding to usermin, then added my domain, then disabled forwarding to usermin.