Trigger event from external domain Google Analytics

I have a shop on a third party service. I want to trigger a event when the product gets purchased.

The third party service allows us to add script snippets.

I’m doing this, but I can’t see any custom Events on Reports > Real Time > Events on Google Analytics:

<!-- Global site tag (gtag.js) - Google Analytics --> <script async src="https://www.googletagmanager.com/gtag/js?id=UA-XXXXX-X"></script> <script>   window.dataLayer = window.dataLayer || [];   function gtag(){dataLayer.push(arguments);}   gtag('js', new Date());    gtag('config', 'UA-XXXXXXXX-X'); //my ID   gtag('send', 'event', 'demo');    </script> 

Am I missing something? Or is this a restriction that won’t allow us to send events from external domains?

To test this out I’m using https://jsfiddle.net/ and codepen.

Update

I’m trying it from an external domain of my own and it doesn’t register the event either. This time I tried with google analytics instead as this is the exact same code I use on my main domain:

    <script src="https://www.google-analytics.com/analytics.js"></script>     <script>                  ga('create', 'UA-XXXXXXXX', 'auto');                  ga('send', 'event', 'myTestEvent', 'submit', {             hitCallback: function() {                 // this gets triggered successfully                  console.log("triggered callback!");             }         });     </script> 

Just to clarify, the analytics ID I’m using is the one from my main domain. I would assume using such ID will trigger all events to that account.