command.php

<?php

namespace Drupal\ton_module\Ajax;

use Drupal\Core\Ajax\CommandInterface;

/**
 * Provides an AJAX command ....
 *
 * This command is implemented in Drupal.AjaxCommands.prototype.ajaxRedirect.
 */
class AjaxRedirectCommand implements CommandInterface {

  /**
   * The url.
   *
   * @var string
   */
  protected $url;

  /**
   * AjaxRedirectCommand constructor.
   *
   * @param $url
   * @param $time
   */
  public function __construct($url) {
    $this->url = $url;
  }

  /**
   * {@inheritdoc}
   */
  public function render() {
    return [
      'command' => 'ajaxRedirect',
      'url' => $this->url,
    ];
  }

}

form.php

<?php

  /**
   * {@inheritdoc}
   */
  public function form(array $form, FormStateInterface $form_state) {
    $form = parent::form($form, $form_state);
    $options = ['foo' => 'foo', 'bar' => 'bar'];
    $form['dummy_radio'] = [
      '#type' => 'radios',
      '#title' => 'Dummy radios',
      '#options' => $options,
      '#attached' => [
        'library' => [
          'my_module/my_command.commands',
        ],
      ],
      '#ajax' => [
        'callback' => [$this, 'myDummyCallback'],
        'event' => 'click',
        'progress' => [
          'type' => 'throbber',
          'message' => 'Wait...',
        ],
      ],
      '#required' => TRUE,
    ];

    return $form;
  }
  
    public function myDummyCallback(array $form, FormStateInterface $form_state) {
    $response = new AjaxResponse();
    $url = 'http://google.fr';
    // TODO SOME STUFF ....
    $response->addCommand(new AjaxRedirectCommand($url));
    return $response;
  }

gistfile1.js

/**
 * @file
 * Handles AJAX.
 */

(function ($, Drupal, drupalSettings) {

  'use strict';

    /**
     * Ajax command for triggering redirect.
     *
     * @param {Drupal.Ajax} [ajax]
     *   An Ajax object.
     * @param {object} [response]
     *   The Ajax response.
     * @param {number} [status]
     *   The HTTP status code.
     */
    Drupal.AjaxCommands.prototype.ajaxRedirect = function (ajax, response, status) {
      window.location = response.url;
    };

})(jQuery, Drupal, drupalSettings);

xxx.libraries.yml

my_command.commands:
  version: VERSION
  js:
    js/gistfile1.js: {}
  dependencies:
     - core/drupal.ajax