Step 1 - In the Outlook desktop app, click 'File'
Step 2 - Choose Account Settings, then select Account Settings from the menu.
Step 3 - Select the Email tab. Make sure your Exchange email account is selected, then choose 'Change...'
Step 4 - Select 'More Settings'
Step 5 - Click the 'Advanced' tab, then click 'Add'.
Step 6 - Type the name of the shared mailbox you want to add, then click OK.
Step 7 - Choose Next > Finish > Close.