Tracking code example for Event Espresso 4. Adds event-specific tracking code to the thank you page if "tracking_value" custom field is set in event.

tracking_code_example.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 ) {
        $registrations = $transaction->registrations();
        $registration = reset($registrations);
        if ( $registration instanceof EE_Registration ) {
            $event = $registration->event();
            if ( $event instanceof EE_Event ) {
                $tracking_value = get_post_meta( $event->ID(), 'tracking_value', true );
                if ( $tracking_value != '' ) {
                    $tracking_code = '<script>/* JavaScript tracking code begin';
                    $tracking_code .= $tracking_value;
                    $tracking_code .= 'end of tracking code */</script>';
                    echo $tracking_code;
                }
            }
        }
    }
}