Thank you for writing.
By default when you press enter the EditAppointmentDialog is closed and this is why the event is fired. This is the default behavior and if you want to enter a new row in the description field you should press Ctrl+Enter.
Also if you want to change this you can create your own descendant of the EditAppointmentDialog and change any properties you want. Detailed information about how this can be done is available in the following article: Adding a custom field to the EditAppointment dialog.
Please let me know if there is something else I can help you with.
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