You can Subscribe to listen for the BeforeSketchCompletedEvent and specify a delegate that will be executed upon receiving the event. To stop listening, you can Unsubscribe.
The following are examples of tools that publish the sketch events - construction tools, Edit Vertices, Align Features, Reshape.
The BeforeSketchCompletedEvent fires when a sketch is finished but before the SketchCompletedEvent and MapTool.OnSketchCompleteAsync. This event can be used to read the current sketch geometry and update it via the BeforeSketchCompletedEventArgs.SetSketchGeometry method. If there are multiple listeners to the event, each gets the latest modified sketch geometry from the previous listener.