PCL Visualizer: Select Area

I m trying to select and area in point cloud display, I have implemented the callback function as well but I m not aware with what key or mouse click combination an area can be selected in PCL visualizer window. My callback is as follows:

void onSelectArea (const pcl :: visualization :: AreaPickingEvent & event, void * args)
{
  std::cout << "[INFO] Area picking event occurred." << std::endl;

  std :: vector < int > indices;
  if (event.getPointsIndices (indices) == false )
      return ;
}

Following is the callback registration command:

viewer_3D->registerAreaPickingCallback(onSelectArea, (void*)&viewer_3D);