final class FormControllerFormActionEvent {
  ....
  protected array $additionalVariables = [];
  
  ...
  
  public function getAdditionalVariables(): array
  {
    return $this->additionalVariables;
  }
  
  public function setAdditionalVariables(array $additionalVariables): void
  {
    $this->additionalVariables = $additionalVariables;
  }
}

//keep this the same
$event = $this->eventDispatcher->dispatch(
            GeneralUtility::makeInstance(FormControllerFormActionEvent::class, $form, $this)
        );

...

$this->view->assignMultiple(
            array_merge(
                [
                    'form' => $form,
                    'ttContentData' => $this->contentObject->data,
                    'messageClass' => $this->messageClass,
                    'action' => ($this->settings['main']['confirmation'] ? 'checkConfirmation' : 'checkCreate'),
                ],
                $event->getAdditionalVariables() 
            )
        );