The "ugly" solution is to add the command for webmin to /etc/rc.local. The commands in this file are executed during systemboot.
A nicer solution is to write an rc.d-script, put it in /etc/rc.d/ and add the script to your /etc/rc.conf.
Should you try to write a script, it's best to start off with one that's already in /etc/rc.d and see how it works.
A bus station is where a bus stops.
A train station is where a train stops.
On my desk I have a workstation.
I took the rc.local way and it works great and is the easiest to me. In case you're wondering how to set it up I posted the line I have in my /etc/rc.local below. Adjust to your install directory of course.