How to add a shared mailbox you are permitted to access to your Outlook
In Outlook for PC - Open and use a shared mailbox in Outlook (microsoft.com)
In Outlook for Mac - Open a shared Mail, Calendar or People folder in Outlook for Mac (microsoft.com)
In Outlook Mobile (Android or IOS) - Add a shared mailbox to Outlook mobile (microsoft.com)
On Outlook web mail - Open and use a shared mailbox in Outlook on the web (microsoft.com)