code: public function makeRedirectPageMandatory(Contao\DataContainer $dc)
  	{
  		$objPage = $this->Database->prepare("SELECT * FROM " . $dc->table . " WHERE id=?")
  								  ->limit(1)
  								  ->execute($dc->id);
  
  		if ($objPage->numRows && $objPage->type == 'logout')
  		{
  			$GLOBALS['TL_DCA']['tl_page']['fields']['jumpTo']['eval']['mandatory'] = true;
  		}
  	}
docs: Make the redirect page mandatory if the page is a logout page
  
  @param Contao\DataContainer $dc
  
  @throws Exception