URL will not work.
Create an email where fep-email-piping.php file is. Then set this email as piping email in Front End PM PRO > Settings > Emails. It does not have to be your main domain. Or you can forward your main domain email(Gsuite) to your temporary email (email where fep-email-piping.php file is) then from this email forward to fep-email-piping.php.