Click on 'Add calendar' under the month view on the left.
Choose 'Add from directory' on the sidebar options.
In the dropdown, select your main email address. In the below screenshot this is censored.
In the 'Enter a name or email address' field, enter the "email address" of the shared calendar. If you are not sure what this is, please check with a colleague or raise a support ticket for further details.
Choose where you want the calendar to be added in Outlook, we suggest leaving it as default ('Other calendars').
Click on 'Add'.
You will now see your new calendar in the left side bar. This can be toggled on/off to suit your preference.