If you log in as the Virtual Server owner, go into Webmin Modules -> Scheduled Commands, and you can setup a command to run there at what ever interval you require.
Depending on whether the cron relies on any of the apache environment variables, you can either use the php commands mentioned already, or you can use the command below to run the script via the url
that will trigger the script, and discard any output that is received, if the script generates a response you want to keep, you can replace /dev/null with the logfile you want to use, and use >> instead of > if you want to have an ongoing log, rather then just the last run