* @ContentEntityType(
 ...
 *   handlers = {
 *     "form" = {
 *       "add" = "Drupalmy_moduleFormMyEntityForm",
 *       "edit" = "Drupalmy_moduleFormMyEntityForm",
 *       "delete" = "Drupalmy_moduleFormMyEntityDeleteForm",
 ...

$form['add_my_entity_button'] = [
  '#type' => 'link',
  '#title' => t('Add NewEntity'),
  '#url' => Url::fromRoute(
    'my_module.add_entity',
    array(
      'node' => is_numeric($node) ? $node : NULL,
    )
  ),
  '#attributes' => [
    'class' => ['button', 'use-ajax'],
    'data-dialog-type' => 'modal',
  ],
  '#attached' => [
    'library' => ['core/drupal.dialog.ajax'],
  ],
];