Add css class to the container of the action button from a Form
Thanks to kint for devel options:

There you go:

/**
 * Implements hook_form_alter().
 */
function twig_block_form_alter(&amp;$form, $form_state, $form_id) {

  if (strpos($form_id, 'contact_message_feedback_form') !== FALSE) {
    $form['actions']['preview']['#access'] = FALSE;
    $form['actions']['submit']['#attributes']['class'][] = 'btn btn-primary btn-xl text-uppercase';
    // kint($form['actions']['submit']);
    $form['actions']['submit']['#prefix'] = '&lt;div class="col-md-12 text-center"&gt;';
    $form['actions']['submit']['#suffix'] = '&lt;/div&gt;';
  }
}