Step 1
With Outlook already open, click File, Add account.
Step 2
Type the email address of the account you'd like to add, followed by the password.
Step 3
You may be asked to "Allow my organisation to manage my device". Click "No, sign in to this app only".
Step 4
Click "Done" and restart Outlook.
Step 5
You should find the new mailbox when scrolling down the list of folders on the left hand pane.