D8 Redirect on registration form

d8_redirect_regstration.php

<?php

/**
 * Implements hook_form_FORM_ID_alter().
 */
function capitalcamp_glue_form_user_register_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // @todo come up with a more elegant approach here.
  // Alter login form and add own custom submit handler.
  $form['actions']['submit']['#submit'][] = '_capitalcamp_glue_user_register_form_submit';
}

/**
 * Custom submit handler for registration form.
 */
function _capitalcamp_glue_user_register_form_submit($form, FormStateInterface $form_state) {
  // Set redirect to login form.
  $url = Url::fromUri('internal:/form/registration');
  $form_state->setRedirectUrl($url);
}