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:
Since launching the Jira Software 8.0 and Jira Service Desk 4.0 platform releases in February, many of you have been asking when the next Enterprise release will be so you can take advantage of the m...
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