Using Classic Outlook on Windows for personal email, calendar, and contact management
Dear Mischelle (US - New York),
Good day!
Thank you for posting to Microsoft Community. We are glad to assist.
As per your mentioned description about "Shared Mailbox - Add on". Shared mailbox doesn't support add in because shared mailbox don’t have a license, so in some cases they do have some difference with individual mailboxes.
At the same time, I would like to share some information with you, Microsoft has specific dedicated support channel for certain different aspect and attributes, would you mind to kindly also post your concern on our specific Microsoft Q&A forum community for certain different support scope? Our relevant community members can further provide you possible information from their side.
Here is that dedicated forum community to place your concern: Microsoft Q&A
Thanks for your understanding! Your patience and cooperation will be highly appreciated. I wish you all the best!
Best regards,
Inema | Microsoft Community Moderator