code: public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
      /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
      $entity = $items->getEntity();
  
      $workflow = $this->moderationInformation->getWorkflowForEntity($entity);
      $default = $items->get($delta)->value ? $workflow->getTypePlugin()->getState($items->get($delta)->value) : $workflow->getTypePlugin()->getInitialState($entity);
  
      /** @var \Drupal\workflows\Transition[] $transitions */
      $transitions = $this->validator->getValidTransitions($entity, $this->currentUser);
  
      $transition_labels = [];
      $default_value = NULL;
      foreach ($transitions as $transition) {
        $transition_to_state = $transition->to();
        $transition_labels[$transition_to_state->id()] = $transition_to_state->label();
        if ($default->id() === $transition_to_state->id()) {
          $default_value = $default->id();
        }
      }
  
      $element = [
        '#type' => 'select',
        '#key_column' => $this->column,
        '#options' => $transition_labels,
        '#default_value' => $default_value,
        '#access' => !empty($transition_labels),
        '#wrapper_attributes' => [
          'class' => ['container-inline'],
        ],
      ];
      $element['#element_validate'][] = [get_class($this), 'validateElement'];
  
      return $element;
    }
docs: {@inheritdoc}