The shortcuts in the planner communicate directly with your built-in iPad calendar to prompt a "quick create" window to appear over your planner. So if the events are not scheduling to the correct calendar, this is likely an issue with the way your calendar is set up on your iPad.
At the end of this video I demonstrate how to make sure your calendar is selected and set as your default calendar.
If you use multiple calendars, you can always scroll down in the calendar "create event" window to change the calendar.
If you're new to using a calendar on your iPad, you may find the Apple user guide helpful: https://support.apple.com/guide/ipad/create-and-edit-events-in-calendar-ipad99d9847f/ipados
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