In order to add tasks to calendar automatically, You have to change settings.
Select "General" in Settings and turn on "Auto Event Generate" switch.
2. Adding Tasks to Calendar Automatically
When you decide due date of some tasks, they will added to calendar.
When you drop a task to "Planned", The Calendar to decide due date will pop up.
The task has been added to calendar.
3. Selecting Default Calendar to Add Tasks
You can select default calendar to add tasks.