ActionAddForm::buildForm

public ActionAddForm::buildForm(array $form, FormStateInterface $form_state, $action_id = NULL)

public function buildForm(array $form, FormStateInterface $form_state, $action_id = NULL) {
  // In \Drupal\action\Form\ActionAdminManageForm::buildForm() the action
  // are hashed. Here we have to decrypt it to find the desired action ID.
  foreach ($this->actionManager->getDefinitions() as $id => $definition) {
    $key = Crypt::hashBase64($id);
    if ($key === $action_id) {
      $this->entity->setPlugin($id);
      // Derive the label and type from the action definition.
      $this->entity->set('label', $definition['label']);
      $this->entity->set('type', $definition['type']);
      break;
    }
  }

  return parent::buildForm($form, $form_state);
}