These instructions are for users who use Microsoft 365 (M365) for their personal email.
The instructions for users whose email account is an on-prem Connect account are available: Adding a shared mailbox to Outlook for Windows - Instructions for M365 Shared Mailboxes. (For users using Connect for their personal email.)

To add a shared mailbox it is necessary to have all of the following:

“Send-as” and/or full access permissions to all folders of the shared mailbox
Your own username requires permission to the shared mailbox. You will login using your own credentials. (Your department IT support can help you request access.)
Permissions on Windows Nexus computers become effective the next time you log onto the machine after the mailbox permission change has been made. You may need to log out from your Nexus account on the machine and then log in again to add the account and access the mailbox.

Contact IST Service Desk if your account does not meet the above requirements.

Not what you’re looking for? Please browse Shared Mailbox articles: Shared Mailboxes

For more information, please also see the article Working with Generic and Shared Mailboxes

Adding a shared M365 account in Outlook for Windows

Open Outlook.
Click on the File tab in the ribbon.
Choose Account Settings then select Account Settings from the drop-down menu.
Select the Email tab.
Make sure your own UWaterloo email account is highlighted then choose Change.
Click on the More Settings button
Select the Advanced tab
Click on the Add button
Enter the 8-character username the name of the shared mailbox:
Click Apply, and then click OK again.
Choose Next
Click Done
Click Close
Close Outlook and re-open it.