Which SR version are you on? Firing a custom event from a listener is already a built in feature.
Go the Script Listeners page and click "Fire an event when condition is true."
You can configure when you want to fire the event and which event to fire.
If you need to create a new custom event, you should see this page in the Atlassian docs:
What is Opsgenie? Opsgenie is a modern incident management platform for operating always-on services that enables dev and ops teams to stay aware and in control of alerts and incidents. Opsgenie at ...
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