Using the Zimbra Web Client (ZWC):
After the calendar has been shared in the ZWC, it can be shared in Outlook and not require credentials.
This example is using Outlook 2016.
The first part is done in ZWC (Zimbra Web Client).
In Outlook:
The shared calendar will now show under Other Calendars in the left pane.
To make the calendar visible you need to check the box next to the shared calendar.