Using webRTC I can capture screen

var myMediaStreamConstraints = {
  video: {
    width: { min: 640, ideal: 1920 },
    height: { min: 400, ideal: 1080 },
    aspectRatio: { ideal: 1.7777777778 }
  },
  audio: {
    sampleSize: 16,
    channelCount: 2
  }
}

var promise = navigator.mediaDevices.getDisplayMedia(myMediaStreamConstraints);

This promise that resolves to a MediaStream containing a video track whose contents come from a user-selected screen area, as well as an optional audio track.

I want to capture only some rectangular part of my screen and I do not want to crop using HTML and CSS thing. I am expecting there would be some way by creating right MediaStreamConstraints.

How can I get cropped MediaStream to capture only rectangular part of screen?