EntityModerationForm::submitForm

public EntityModerationForm::submitForm(array &$form, FormStateInterface $form_state)

public function submitForm(array &$form, FormStateInterface $form_state) {
  /** @var ContentEntityInterface $entity */
  $entity = $form_state->get('entity');

  $new_state = $form_state->getValue('new_state');

  // @todo should we just just be updating the content moderation state
  //   entity? That would prevent setting the revision log.
  $entity->moderation_state->target_id = $new_state;
  $entity->revision_log = $form_state->getValue('revision_log');

  $entity->save();

  drupal_set_message($this->t('The moderation state has been updated.'));

  /** @var \Drupal\content_moderation\Entity\ModerationState $state */
  $state = $this->entityTypeManager->getStorage('moderation_state')->load($new_state);

  // The page we're on likely won't be visible if we just set the entity to
  // the default state, as we hide that latest-revision tab if there is no
  // forward revision. Redirect to the canonical URL instead, since that will
  // still exist.
  if ($state->isDefaultRevisionState()) {
    $form_state->setRedirectUrl($entity->toUrl('canonical'));
  }
}