Log into Office 365 OWA by navigating to https://portal.office.com

Login with your email and password and Click the Mail tile.

On the left navigation pane, under folders click 'More'.

Right click on your name.

Click 'Add shared folder'

Enter the email address of the shared mailbox (e.g. Shared@outlook.com)

You may need to click "Search Directory" to find the mailbox account

Once you find the mailbox click Add.