Redirect to page after Simple News subscription
You do as follows:


  use Drupal\Core\Url;


function MODULENAME_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'FORM_ID') {
    foreach (array_keys($form['actions']) as $action) {
        if ($action != 'preview' &amp;&amp; isset($form['actions'][$action]['#type']) &amp;&amp; $form['actions'][$action]['#type'] === 'submit') {
          $form['actions'][$action]['#submit'][] = '_redirect_submit';
        }
    }
  }
}
/**
 * Add redirect to user after subscription.
 */
 function _redirect_submit(array &amp;$form, FormStateInterface $form_state) {
    //If you know route name you can do like this
    $form_state-&gt;setRedirect(ROUTE_NAME); // example as "user.admin_create" redirects to user create page
    // If you have path do like below.
    $destination = "INTERNAL_URL";
    $url = Url::fromUri('internal:' . $destination);
    $form_state-&gt;setRedirectUrl($url);
 }