SearchPageForm::form

public SearchPageForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $plugin = $this->entity->getPlugin();
  $form_state->set('search_page_id', $this->entity->id());

  $form['basic'] = array(
    '#type' => 'container',
    '#attributes' => array(
      'class' => array('container-inline'),
    ),
  );
  $form['basic']['keys'] = array(
    '#type' => 'search',
    '#title' => $this->t('Enter your keywords'),
    '#default_value' => $plugin->getKeywords(),
    '#size' => 30,
    '#maxlength' => 255,
  );

  // processed_keys is used to coordinate keyword passing between other forms
  // that hook into the basic search form.
  $form['basic']['processed_keys'] = array(
    '#type' => 'value',
    '#value' => '',
  );
  $form['basic']['submit'] = array(
    '#type' => 'submit',
    '#value' => $this->t('Search'),
  );

  $form['help_link'] = array(
    '#type' => 'link',
    '#url' => new Url('search.help_' . $this->entity->id()),
    '#title' => $this->t('Search help'),
    '#options' => array('attributes' => array('class' => 'search-help-link')),
  );

  // Allow the plugin to add to or alter the search form.
  $plugin->searchFormAlter($form, $form_state);

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