1. Make sure the user has Full Access to the Shared Mailbox
2. Log into outlook.office.com (OWA) in a web browser
3. Click on the users' profile picture/initials in the upper right hand corner of OWA and select "Open another mailbox":
4. Type in the name of the Shared Mailbox, select the account, and then click Open:
5. The Shared Mailbox will open. Click on the gear icon in the upper right hand corner of OWA and then "View all Outlook settings":
6. Click on Mail > Rules, and then you can add or edit any Rules for the Shared Mailbox: