Alter an existing form
I had to do the exact same thing yesterday, and I am still looking for a more Drupal 8 way to do this, but haven't found it. I ended up doing it this way:

&lt;?php

/**
 * @file
 * This is my module.
 */

use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Implements hook_form_alter().
 */
function MODULE_form_alter(array &amp;$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'user_form') {
    $form['actions']['submit']['#submit'][] = '_MODULE_goto';
  }
}

/**
 * Make the form go to route.id.
 */
function _MODULE_goto($form, FormStateInterface $form_state) {
  $form_state-&gt;setRedirect('route.id');
}


I am still very interested in doing it a different way, my module looked so clean without the .module file :).