Why can't I make the BBLink calendar on my Z10 the default calendar for new appointments?

When I go enter an appointment on my Z10, it always defaults to the local calendar, so I have to change it to the BBLink/linked computer each time (because the local calendar won't sync with Outlook so I need any new appointments to be on the BBLink calendar).

In calendar app, swipe down from the top, manage
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uncheck the calendars you don't want. as you see i only use my outlook calendar. In your case only check the BB link calendar.
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When you add an appointment to your calendar select the calender you want to use, in your case the BB Link calendar
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To set your preferred calendar as the default, go to Settings, Acounts. From the overflow menu (3 dots at bottom right) tap "Set Defaults". Select the preferred calendar. All new entries will go to that calendar.