class SearchPageAddForm

class SearchPageAddForm extends SearchPageFormBase {
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, $search_plugin_id = NULL) {
    $this->entity
      ->setPlugin($search_plugin_id);
    $definition = $this->entity
      ->getPlugin()
      ->getPluginDefinition();
    $this->entity
      ->set('label', $definition['title']);
    return parent::buildForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  protected function actions(array $form, FormStateInterface $form_state) {
    $actions = parent::actions($form, $form_state);
    $actions['submit']['#value'] = $this->t('Save');
    return $actions;
  }
  
  /**
   * {@inheritdoc}
   */
  public function save(array $form, FormStateInterface $form_state) {
    // If there is no default search page, make the added search the default.
    // TRICKY: ::getDefaultSearchPage() will return the first active search page
    // as the default if no explicit default is configured in `search.settings`.
    // That's why this must be checked *before* saving the form.
    $make_default = !$this->searchPageRepository
      ->getDefaultSearchPage();
    parent::save($form, $form_state);
    if ($make_default) {
      $this->searchPageRepository
        ->setDefaultSearchPage($this->entity);
    }
    $this->messenger()
      ->addStatus($this->t('The %label search page has been added.', [
      '%label' => $this->entity
        ->label(),
    ]));
  }

}