mymodule_form_user_login_form_alter.php

Using Form Alter on Existing Forms - Adding a Blurb to the Drupal Log in Page

/**
 * Implements hook_form_FORM_ID_alter() for form--user-login-form.html.twig.
 */
function sfp_form_user_login_form_alter(&$form, $form_state) {
  $config = \Drupal::config('system.site');
  $site_login_blurb_display = $config->get('site_login_blurb_display');
  $site_login_blurb = ($site_login_blurb_display) ? $config->get('site_login_blurb') : '';

  $form['#prefix'] = sprintf('<div class="site_login_blurb">%s</div>', $site_login_blurb);
  $form['#suffix'] = '';
  if ($site_login_blurb_display) {
    $form['#attached']['library'][] = 'sfp/sfp-libraries';
  }
}