views_autocomplete_api-8.x-1.x-dev/modules/views_autocomplete_api_demo/views_autocomplete_api_demo.module

<?php

/**
 * @file
 * Contain the module of views autocomplete API demo.
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function views_autocomplete_api_demo_form_search_block_form_alter(&$form, FormStateInterface $form_state) {

  // Search on multiple views.
  $view_name = 'search_auto_publish,search_auto_unpublish';
  // You can omit display just put empty value with comma separator.
  $display_id = ',block_1';
  // @todo needs reviews
  // $views_arguments='args1+arg2,arg3';
  // Change the type of search field to textfield to enable autocompletion.
  $form['keys']['#type'] = 'textfield';
  // The autocomplete route name must be usually views_autocomplete_api.
  $form['keys']['#autocomplete_route_name'] = 'views_autocomplete_api';
  // The parameters must be passed ou #autocomplete_route_parameters with a
  // valid view's machine name.
  $form['keys']['#autocomplete_route_parameters'] = [
    'view_name' => $view_name,
    'display_id' => $display_id,
  ];

}