EntityModerationForm::buildForm

public EntityModerationForm::buildForm(array $form, FormStateInterface $form_state, ContentEntityInterface $entity = NULL)

public function buildForm(array $form, FormStateInterface $form_state, ContentEntityInterface $entity = NULL) {
  /** @var \Drupal\content_moderation\Entity\ModerationState $current_state */
  $current_state = $entity->moderation_state->entity;

  $transitions = $this->validation->getValidTransitions($entity, $this->currentUser());

  // Exclude self-transitions.
  $transitions = array_filter($transitions, function(ModerationStateTransition $transition) use ($current_state) {
    return $transition->getToState() != $current_state->id();
  });

  $target_states = [];
  /** @var ModerationStateTransition $transition */
  foreach ($transitions as $transition) {
    $target_states[$transition->getToState()] = $transition->label();
  }

  if (!count($target_states)) {
    return $form;
  }

  if ($current_state) {
    $form['current'] = [
      '#type' => 'item',
      '#title' => $this->t('Status'),
      '#markup' => $current_state->label(),
    ];
  }

  // Persist the entity so we can access it in the submit handler.
  $form_state->set('entity', $entity);

  $form['new_state'] = [
    '#type' => 'select',
    '#title' => $this->t('Moderate'),
    '#options' => $target_states,
  ];

  $form['revision_log'] = [
    '#type' => 'textfield',
    '#title' => $this->t('Log message'),
    '#size' => 30,
  ];

  $form['submit'] = [
    '#type' => 'submit',
    '#value' => $this->t('Apply'),
  ];

  $form['#theme'] = ['entity_moderation_form'];

  return $form;
}