Is your alert Javascript code wrapped in an opening and closing script tag? It would appear like the below:
<script type=“text/javascript” src=“https://uploads-ssl.webflow.com/5fd13b6440d790c78d867ae4/60b5562027813d45e349b4f2_text.txt”></script>
<script>
$( document ).ready(function() {
alert(“Hello! I am an alert box!!”);
});
</script>