$info = $form_state->getBuildInfo();

$form['#prefix'] = '<section class="hl__single-input-form">';
$form['#suffix'] = '</section>';
$form['#attributes']['class'][] = 'hl__single-input-form__form';

$form['title'] = [
  '#type' => 'label',
  '#for' => 'edit-email-address',
  '#title' => 'Stay in the know',
  '#attributes' => [
    'class' => [
      'hl__label',
      'hl__label--inline',
    ],
  ],
];

if (!empty($info['args'][0]) && $info['args'][0] == "show_tagline") {
  $form['tagline'] = [
    '#markup' => '<div class="hl__single-input-form__helper-text">Sign up for email updates from MySite</div>',
  ];
}

$form['email_address'] = [
  '#type' => 'email',
  '#size' => NULL,
  '#required' => TRUE,
  '#attributes' => [
    'placeholder' => $this->t('Email'),
    'class' => [
      'hl__input',
      'hl__input--inline',
      'js-is-required',
    ],
    'data-twig-suggestion' => 'newsletter_signup_email',
  ],
  '#prefix' => '<div class="hl__single-input-form__input-group">',
];

$form['submit'] = [
  '#type' => 'submit',
  '#value' => $this->t('Sign up'),
  '#attributes' => [
    'class' => [
      'hl__button',
      'hl__button--small',
      'hl__button--inline',
    ],
    'data-twig-suggestion' => 'newsletter_signup_submit',
  ],
  '#suffix' => '</div>',
  '#ajax' => [
    'wrapper' => 'hl__single-input-form',
  ]
];

/**
   * {@inheritdoc}
   */
  public function build() {
    return ['form' => $this->formBuilder->getForm('Drupalharlib_newsletter_signupFormNewsletterSignup')];
  }

$variables['signup_form'] = Drupal::service('form_builder')->getForm('Drupalharlib_newsletter_signupFormNewsletterSignup', "show_tagline");