How to create the Popup or modal in Drupal 8?

The following code will help you to create the popup in Drupal 8. 

use \Drupal\Core\Ajax;

function testingPopup() {
  $response = new AjaxResponse();
  $title = $this->t('Title for the Popup.');
  $form['#attached']['library'][] = 'core/drupal.dialog.ajax';
  $response->setAttachments($form['#attached']);
  $content = '<div class="test-popup-content">' . 'Content in HTML format' . '</div>';
  $options = array(
    'dialogClass' => 'popup-dialog-class',
    'width' => '75%',
  );
  $modal = new OpenModalDialogCommand($title, $content, $options);
  $response->addCommand($modal);
  return $response;
}

You can call the function testingPopup() , you will get the popup.