Adding PHP line in the child theme?

I read the in the documentation here, however, I still don't know how to accomplish what I want.

I do have installed and activated GP Child theme and I want to add 1 line of code to display breadcrumbs in my posts.

<?php if (function_exists('rank_math_the_breadcrumbs')) rank_math_the_breadcrumbs();

My style.css and functions.php are both empty. How do I add that line and where? The point is not to lose it after GP is update (hence why I have the child theme)