What is this?
How to add shared mailbox in Outlook on the Web (Webmail)
Instructions
After you complete this procedure, the shared mailbox for which you have been provided access to will display in your folder list every time you open Outlook on the Web.
Log in to your mailbox using Outlook on the Web (Webmail).
Right-click on the word 'Folders' on the left side of the page, then click Add shared folder.
In the Add shared folder dialog box, type the name of the mailbox that you have been provided access, and click Add.
The mailbox appears in your Outlook on the Web folder list.
If you decide that you no longer want to see the other person’s mailbox every time you open Outlook on the Web, right-click the folder, and click Remove shared folder.
Note: If the shared mailbox was added for you by Digital Innovation & Technology Services, you may have a submit a ticket to remove the shared folder.
https://support.office.com/en-us/article/access-another-person-s-mailbox-a909ad30-e413-40b5-a487-0ea70b763081#__toc372388945