I have 2 apps that have been running in Sierra. Both require access to the calendar. Both work with no issue under Sierra. Now I have compiled both under Catalina. While the one still works, the other does not appear in the Settings/.../Calendar. I don't see any option to add my app to that list. Resource Access for Calendar is ticked. The only difference between those apps is that one is has a GUI while the one not working is made for the menu bar.
nwm It appears that NSCalendarsUsageDescription must be added to Info Plist. Welcome to Vista.