newsletter-bundle/src/Resources/contao/dca/tl_newsletter.php
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 $varValue;
	}
public function addSenderPlaceholder($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;
	}
Add the sender address as placeholder

@param mixed                $varValue
@param Contao\DataContainer $dc

@return mixed