code: public function addSenderPlaceholder($varValue, Contao\DataContainer $dc)
  	{
  		if ($dc->activeRecord && $dc->activeRecord->pid)
  		{
  			$objChannel = $this->Database->prepare("SELECT sender FROM tl_newsletter_channel WHERE id=?")
  										 ->execute($dc->activeRecord->pid);
  
  			$GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['eval']['placeholder'] = $objChannel->sender;
  		}
  
  		return $varValue;
  	}
docs: Add the sender address as placeholder
  
  @param mixed                $varValue
  @param Contao\DataContainer $dc
  
  @return mixed