The phpbb event list contains a typo in ucp_pm_view_messsage (triple S instead of double). Using the correctly spelled version won't work as the event trigger itself is spelled wrong.
This works:
'core.ucp_pm_view_me sss age' => 'addMessageId'
This doesn't
'core.ucp_pm_view_me ss age' => 'addMessageId'
(spaces only for easier readability)