scroller_class_opform.php

public function buildOptionsForm(&$form, FormStateInterface $form_state) {
  parent::buildOptionsForm($form, $form_state);

  $form['height'] = array(
    '#type' => 'textfield',
    '#title' => $this->t('Height'),
    '#description' => $this->t('Default height of header.'),
    '#size' => '6',
    '#default_value' => $this->options['height'],
    '#required' => TRUE,
    '#field_suffix' => 'px',
  );
}