In Jira DC Automation the Multiple Issue Events trigger can be used to trigger an Automation Rule based on a Custom Event. In Jira Cloud the list of selectable events for that trigger doesn't include custom events and does not even include all System defined events.

Customer would like for the multiple issue events trigger to be able to be triggered based on custom events

AUTO-83 seemed to similarly cover this but was closed as a duplicate of JSDCLOUD-12358 without any reasoning given. However the two are unrelated as JSDCLOUD-12358 covers "Automation - Improve Edit Issue Action to include other fields, e.g. Organisation, Resolution field or custom fields" and does not cover triggers or custom events.