When using Teamup in a browser, the event editor does not pre-select a sub-calendar. However, after you've created an event, the calendar remembers the last choice and pre-selects the same sub-calendar(s) again when you create a new event. This assumes that often the same sub-calendars are used again and again.
Here's a shortcut to quickly show your preferred sub-calendar (rather than pre-selected sub-calendars from your last choice):
From the sidebar, tap the Eye icon of the preferred sub-calendar so it is the only calendar displayed.
When you open the event editor, that sub-calendar will be selected already.
To remove any pre-selected options from the event editor, refresh the page before creating a new event.