Shutdown of the appliance can be scripted through the Maintenance->Webmin->Shutdown https interface. Here's an example using the freely available "wget" utility (all on one line):
wget --output-document=/dev/null --no-check-certificate --post-data="user=admin&pass=ADMINPASSWORD&confirm=Shutdown+System" https://APPLIANCE:999/ https://APPLIANCE:999/session_login.cgi https://APPLIANCE:999/init/shutdown.cgi
In the above replace "ADMINPASSWORD" with the password for your appliance's "admin" account. Be sure to URL escape any special characters like space, &, and % (%20, %26, and %25 respectively).
Replace "APPLIANCE" with the hostname or IP of your appliance.
wget --output-document=/dev/null --no-check-certificate --post-data="user=admin&pass=ADMINPASSWORD&confirm=Shutdown+System" https://APPLIANCE:999/ https://APPLIANCE:999/session_login.cgi https://APPLIANCE:999/init/shutdown.cgi
In the above replace "ADMINPASSWORD" with the password for your appliance's "admin" account. Be sure to URL escape any special characters like space, &, and % (%20, %26, and %25 respectively).
Replace "APPLIANCE" with the hostname or IP of your appliance.