Freemius Purchase Completion JavaScript Callback Filter

purchaseCompleted.php

<?php
    // Add GA tracking only if user opted-in OR if non-WP.org compliant product.
    function my_after_purchase_js( $js_function ) {
        return 'function ( response ) {
            /**
             * Since the user just entered their personal & billing information, agreed to the TOS & privacy,
             * know they are running within a secure iframe from an external domain, they implicitly permit tracking
             * this purchase. So initizlizing GA here (after the purchase), is legitimate.
             */
            ga('create', 'UA-XXXXXXX', 'auto');
            
            console.log("checkout", "purchaseCompleted");
        }';
    }  

    function my_checkout_enrich ( $html ) {
        return '<script type="text/javascript">
            (function() {
                if ( null == ga ) {
                    // Add code to include GA.
                }
            })();
        </script>' . $html;
    }

    my_freemius()->add_filter('checkout/purchaseCompleted', 'my_after_purchase_js');
    my_freemius()->add_filter('templates/checkout.php', 'my_checkout_enrich');
?>

@hqwebs This might be a silly question, but where should I add this code?