Adding Shared Mailboxes in Outlook Web

Shared Mailboxes in the Desktop Outlook Application
When using the desktop version of Outlook, shared mailboxes are added automatically. However, if you're using Outlook Web (outlook.office.com), it is necessary to manually add shared mailboxes in order to access them. This process is simple but requires a few steps, which I have outlined in detail below.

Log in to Outlook Web

Open your web browser and go to http://Outlook.com .
Log in using your Microsoft Office 365 credentials.
After logging in, click on the Mail icon (small envelope) located at the bottom left of the screen.

Expand Folders
On the left side of the screen, you will see a list of folders, such as Inbox, Sent Items, Drafts, etc. If necessary, scroll up or down through the folder list until you see the Folders section.
Click on the three dots (...) next to the word FOLDERS.
In the expanded menu, select the option Add shared folder.

In the window that appears, enter the name or email address of the shared mailbox you want to add.
Outlook will suggest available options as you start typing the name. Select the correct mailbox from the list.

Confirm the addition of the mailbox. After selecting the correct mailbox, click Add

The shared mailbox will be added to the folder list and will appear in your sidebar navigation.
Done! Now you have access to the shared mailbox through Outlook Web and can manage messages and other items in that mailbox just like in your own inbox.

If you do not have the appropriate permissions or have not been invited to use the shared mailbox, you will not be able to add it in Outlook Web.

Shared mailboxes are only available to users who have been assigned to them by the administrator or the mailbox owner.