Edit in JSFiddle

const playbtn = document.querySelector('button');
  const divBox = document.querySelector('div');
  const video = document.querySelector('video');
  
  playbtn.onclick = function() {
    divBox.setAttribute('class','showing');
  }

  divBox.onclick = function() {
    divBox.setAttribute('class','hidden');
  };

  video.onclick = function() {
    video.play();
  };

<button>Play Video</button>

<div class="hidden">
  <video width="320" height="240">
    <source src="http://dglobaltech.com/wp-content/uploads/2021/01/Coding-Interview-Question_-Two-Sum-Target.mp4" type="video/mp4">
    <source src="http://dglobaltech.com/wp-content/uploads/2021/01/Coding-Interview-Question_-Two-Sum-Target.ogg" type="video/ogg">
Your browser does not support the video tag.
  </video>
</div>

.hidden {
  display: none;
}

.showing {
  display: block;
}