The 'quick' means to create a meeting request in the Outlook calendar does not trigger the send event in any Outlook web add-ins, for details see issue - https://github.com/OfficeDev/office-js/issues/4074
The send event is raised if the meeting organizer switches to use the full compose mode which means compliance-mode Outlook web add-ins can then check the meeting request content before it is actually sent.
This feature request is that M365 administrators have the ability to configure organization users to always use the full compose mode when creating meeting requests from the 'quick-event' option so that Outlook on-send add-ins are successfully triggered upon the sending of the meeting request. An alternative, but undesirable option would be to have an administration setting that disables the 'quick-event' interface altogether. If either of these M365 administration option was implemented, ideally it could be scoped to the entire organization, AD security groups or individuals; in the same way that Outlook add-ins can be scoped.