if (array_key_exists('search_broadness', $get_results) && $get_results['search_broadness'] == 'narrow') {
  $block .= 'data-as_sitesearch="' . $gcse_narrow_search_query . '"';
}
$form['search']['results'] = array(
  '#type' => 'item',
  '#markup' => $block,
  '#allowed_tags' => ['script', 'div'],
)

<div class="gcse-searchresults-only" sitesearch="https://www.example.com/drupal8_gcse/"></div>