To start, log in to webmail and click on the Calendar tab.
In the drop-down menu next to Public Calendar choose Full details
Again, right-click on your calendar and choose permissions...
Right-click on Subscribe to Calendar
and choose Copy shortcut
Paste that link into Notepad or some other program and you should see that it looks something like
If your link doesn't have an "s" in "https" at the beginning, you'll need to add it in.
You now have a link you can paste in to your other calendar program. For Google Calendar, follow the instructions here: https://support.google.com/calendar/answer/37100?hl=en