The below video and proceeding guide will show you how to open another mailbox in the Outlook Web App.
You can access the Outlook Web App by going to outlook.office.com.
You must have permissions to the other mailbox to open it.
Select your Account profile icon, in the top right corner. Then select Open Another Mailbox.
This will open another window. Enter the name or email address of the mailbox in the search, and select it. Then select Open.
The other mailbox will open in a new tab.
Another option is to click the ellipses button next to your mailbox within Outlook Web and select "Add Shared folder or mailbox"
This will add the mailbox to your Folder List within Outlook Web.
For more information, please visit the Microsoft 365 Resources on the Patriot Knowledge Center.