Not possible with just GDScript afaik. But a workaround is not that hard.
Define a CollisionPolygon. Then in the insepctor for it, you set "Pickable" to true. This way you will get input events when you click on the shape, which you can catch with
func _input_event( Object viewport, InputEvent event, int shape_idx ):
if shape_idx == idx_of_your_shape: