In order to get the DayStartTime and DayEndTime of the ActiveViewDefinition you will need to firstly bind the ActiveViewDefinition property of the ScheduleView to a property in the ViewModel. Then you will need to add an additional property of CustomSlotSelectionBehavior which will hold the reference of the ViewModel which afterward will be used in order to get the desired properties of the ActiveViewDefinition. The final step will be to define the ViewModel as Resource in XAML so you can pass it to the property as StaticResource. Please check the attached sample project which demonstrates the exact approach.
Hope this helps.