Example of a custom function to add tacking to the thank you page.

my_custom_tracking_code_function.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action(
    'AHEE__thank_you_page_overview_template__top',
    'my_custom_tracking_code_function'
);
function my_custom_tracking_code_function($transaction) {
    if ( $transaction instanceof EE_Transaction ) {
        $tracking_code = 
        "<!– Event snippet for Purchase conversion page –>
            <script>
            gtag('event', 'conversion', {
                'send_to': 'XX-XXXXXXXX/XXXXXXXX-XXXXXXXXX',
                'value': {$transaction->total()},
                'currency': 'AUD',
                'transaction_id': {$transaction->ID()}            
            });
            </script>
        ";
        echo $tracking_code;
    }
}