Issue using gtag code

I have followed the steps listed here for using Google Analytics: https://contactform7.com/tracking-form-submissions-with-google-analytics/

Here is my code in my functions file:

<?php
add_action( 'wp_header', 'mycustom_wp_header' );
 
function mycustom_wp_header() {
?>
<script type="text/javascript">
document.addEventListener( 'wpcf7mailsent', function( event ) {
    gtag('event', 'conversion', {'send_to': 'XXXXXXXXXXXX'});

}, false );
</script>
<?php
}
?>

I cannot get the gtag to work (track form submissions). I was able to get ga to work fine. Any suggestions would be greatly appreciated, thanks!