How to open a Shared Mailbox using Outlook Web Access:
Firstly, you will need to sign in to office.com with your Office 365 account. Once you have logged in you will see a list of applications, you will need to open Outlook.
When in Outlook Web Access, you will need to select the profile icon at the top right corner of the screen. From the menu you need to select Open another mailbox.
Once you have select Open another mailbox, you can then enter the desired Shared Mailbox and then click open.
You have now opened the shared mailbox.