How should I perform a redirect in hook_form_alter()?
use Drupal\Core\Form\FormStateInterface;

/** 
 * Implements hook_form_FORM_ID_alter().
 */
function [MODULENAME]_form_user_login_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  // Alter login form and add own custom submit handler.
  $form['#submit'][] = '_[MODULENAME]_user_login_form_submit';

  // Not all forms pay attention to $form['#submit'][]. For these
  // try adding your submit hook to the #submit for the specific
  // button.  e.g.,
  $form['actions']['submit']['#submit'][] = '_[MODULENAME]_user_login_form_submit';
}

/**
 * Custom submit handler for login form.
 */
function _[MODULENAME]_user_login_form_submit($form, FormStateInterface $form_state) {
  // Set redirect to login form.
  $form_state-&gt;setRedirect('custom.redirect_callback');
}