Where and how create an DOM event @ website?

Hiya,

I had an questionaire from a local marketing agency about Thank You Page-submissions to track visitors If they used the contactform at the client’s website and they are using Google Analytics.
But, I found this outdated thing: Thank You-pages.

I was reading upon your website: https://contactform7.com/redirecting-to-another-url-after-submissions/

I saw the JavaScript-code, but where i need to paste it.

<script>
document.addEventListener( 'wpcf7mailsent', function( event ) {
    location = 'https://xxxxxxxxxx.tld';
}, false );
</script>

And I saw also this one: https://contactform7.com/tracking-form-submissions-with-google-analytics/

What is the best way to track with Google Analytics?

Thanks in advance!

Sincerley,
Bas