Adding a Shared Mailbox on your iOS device (iPhone, iPad)

Step-by-step

Go to Settings > Mail > Add account (in iOS 11-13 go to Settings > Accounts & Passwords > Add account) (iOS 14 - Settings > Mail > Accounts > Add Account)
Select Other from the list of types.
Tap Add Mail Account.
Enter a name for the mail account (e.g. IT Shared Mailbox)
Enter the email address of the shared mailbox
Enter the password of a mailbox that has access to the shared mailbox (e.g. your own password)
Enter a description for the mailbox (e.g. Shared Departmental Mailbox)
Tap Next.
Select IMAP
Fill in the following:
Under Incoming Mail Server
Host Name: outlook.office365.com
User Name: Username@wearebw.com\sharedmailbox@wearebw.com
Password: Your email’s password

Under Outgoing Mail Server
Host Name: smtp.office365.com
User name: YourUsername@wearebw.com
Password: Your email’s password
Tap Next, then tap Save.
In the next screen, choose which items to sync (generally just emails)
Tap Save.
Exit Settings and start the Mail app.