Basically, it uses configd, so you have to create a service definition for your new script.
Create a file with the name actions_NAME.conf, where NAME is something meaningful to you. The file should have a basic win ini format, with the action needed, then a bunch of lines describing what to do.
Example of a file named actions_updatedns.conf:
[reload] command:/root/updateDNS parameter: type:script message:update Daily Data DNS description:Update Daily Data DNS
Now that you have added a new config, you need to reload configd so it will read it:
service configd restart
And, test that you did everything right
configctl updatedns reload
the script /root/updateDNS will be run. You can create multiple actions (stop,start) in the same file with different scripts and/or parameters.
Once this is done, the string after message (or description, I don't know which) will show up as a possible cron job in the opnSense GUI