Redirect user after register form submit to internal page.

snippet.php

/**
 * Implements hook_form_BASE_FORM_ID_alter().
 */
function ns_user_form_user_register_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form['actions']['submit']['#value'] = t('Send');
  $form['#attached']['library'][] = 'core/jquery.ui.tooltip';
  $form['#attached']['library'][] = 'nedapsecurity/tooltips';
  $form['actions']['submit']['#submit'][] = 'ns_user_register_submit_handler';
}

/**
 * Custom submit handler.
 */
function ns_user_register_submit_handler(&$form, FormStateInterface $form_state) {
  $url = Url::fromUri('internal:/thank-you');
  $form_state->setRedirectUrl($url);
}