public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['subject'] = [
      '#type' => 'textfield',
      '#title' => t('Subject'),
      '#required' => TRUE,
      '#default_value' => $form_state->getValue('subject'),
      '#maxlength' => '254',
    ];

    $form['message'] = [
      '#type' => 'textarea',
      '#title' => t('Message'),
      '#required' => TRUE,
      '#default_value' => $form_state->getValue('message'),
      '#cols' => '80',
      '#rows' => '20',
    ];

    $selected_count = $this->context['selected_count'];
    $form['#title'] = $this->t('Send an email to :selected_count members', [':selected_count' => $selected_count]);
    if (isset($form['list'])) {
      unset($form['list']);
    }
    $form['actions']['submit']['#value'] = $this->t('Send email');
    $form['actions']['submit']['#attributes']['class'] = ['button button--primary js-form-submit form-submit btn js-form-submit btn-raised btn-primary waves-effect waves-btn waves-light'];
    $form['actions']['cancel']['#attributes']['class'] = ['button button--danger btn btn-flat waves-effect waves-btn'];
    return $form;
  }