POST_EVENT can take a variable containing the name of the event
to post. (Regardless of what the documentation states).
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What documentation did you use to figure this out?

Check the Language Reference for POST_EVENT statement. I don't see how the
event ID can be created dynamically using the concat operator you used
above. All event ID's must be statically compiled into the trigger code as
far as I can see. Therein lies the problem we have been facing. Am I right
about this?