Pre-requisite: You need to have already set up your main account in Outlook first.
To add a shared mailbox, the steps are very straight forward. Just click on the Home button in the top left hand corner, then click on Add Shared Mailbox.
Enter the email address of the mailbox you have access to. You will be asked to sign in – use your own email account.
The mailbox will then display in the list of accounts.