Editing a php file to add a plugin function

Hi,
I have added a product design plugin to my site. I am supposed to add this to follow this for some of the plugin functions:

1. Add action hook in template

First of all you need to check if your activated theme has a custom content-single-product.php in the woocommerce directory. This directory is in the root folder of your theme. If there is a content-single-product.php, you have to open this one, otherwise go to the woocommerce plugins directory and open the content-single-product.php in the templates directory.

Add following code anywhere in the content-single-product.php, where the product designer should appear:
<?php do_action( ‘fpd_product_designer’ ); ?>

I’m a little nervous about placing the code. I am using the child theme so that I won’t lose anything. Is there a custom area where I can add plugin custom code?