code: public function handle(GetOptionsEvent $event)
      {
          if (('tl_metamodel_dca_combine' !== $event->getEnvironment()->getDataDefinition()->getName())
              || ('rows' !== $event->getPropertyName())
              || !in_array($event->getSubPropertyName(), ['be_group', 'fe_group'])) {
              return;
          }
  
          $isBackend = 'be_group' === $event->getSubPropertyName();
  
          $groups = $this
              ->connection
              ->createQueryBuilder()
              ->select('id')
              ->addSelect('name')
              ->from($isBackend ? 'tl_user_group' : 'tl_member_group')
              ->execute()
              ->fetchAll(\PDO::FETCH_ASSOC);
  
          $result     = [];
          $result[-1] = $event->getEnvironment()->getTranslator()->translate(
              $isBackend ? 'sysadmin' : 'anonymous',
              'tl_metamodel_dca_combine'
          );
  
          foreach ($groups as $group) {
              $result[$group['id']] = $group['name'];
          }
  
          $event->setOptions($result);
      }
docs: Get all options for the user groups.
  
  @param GetOptionsEvent $event The event.
  
  @return void