child theme php

I want to add a php file to my platform child theme so as to allow a featured image, I can change the parent theme but would prefer to add the line of code to my child theme so as to avoid having to do this every time platform is updated.

The parent theme file is core.init.php (the longer version of this file as platform oddly has two such folders, not the one with predicated…)

How do I structure / write this line of php code in a new file using Notepad so that I can upload to my platform child folder on my server?

What do I call it that file?

My child theme folder only has a style.css file in it

Thanks

Ian