Freemius Purchase Completion JavaScript Callback Filter

purchaseCompleted.php

<?php
    function my_after_purchase_js( $js_function ) {
        return 'function (data) {
            console.log("checkout", "purchaseCompleted");
        }';
    }  
    
    my_freemius()->add_filter('checkout/purchaseCompleted', ''my_after_purchase_js);
?>

Hi,
would you elaborate where does this code goes. shell i add this code to the plugin header where Facebook pixel code already added or somewhere else?
Many thanks
irfan Qasim