Turn on the Author button to switch to Author mode.

Select the Body, choose Interaction > Handles > Push.

In the Interaction Editor, click the down arrow on MouseSensor(Body) to expand the trigger list. You will see the Push reaction is connected to the OnLeftClick event.

Turn on the Browse button or press Ctrl+B to preview the interaction.
Click the mouse with the left mouse button over the Body and the surface will be pushed in!