Simple forum search and you can find the answer
https://forum.x-cart.com/showpost.php?p=360239&postcount=2
see after "If using v4.5.5+, you'll need to edit include/templater/templater.php"
Oh sorry I just saw this is for XC5? You cannot do this there. You should create custom module, do all the work in php and call function to return result in template file