We are using OnMessageSend LaunchEvent and we are trying to open web add-in when OnMessageSend Event fired but we are not able to open outlook add-in task pane functionally.
AutoOpen feature is still not available in outlook web add-in.
Please also check raised git ticket as below-
https://github.com/OfficeDev/office-js/issues/3445
Please consider this feature when you go through the planning process.
Thanks.