Embed JS directly in a form

You can in fact use a Rich Text area.

The problem isn't actually the CDATA wrapper -- that's outdated, but harmless, and doesn't stop the code from running.

The problem is that because of the way a form is built out in the DOM, your script will be injected and run twice (which can cause really bad bugs).

So you need to take a rather primitive approach to making sure the function in a given RT area runs only once: name the function uniquely (inside the code block) and note the fact that it's already been run to a global object.

<script>
window.MktoForms2BehaviorsRunCache = window.MktoForms2BehaviorsRunCache || {};
(function(){
  var thisBehavior = "behavior1"; // choose unique function name
  if (thisBehavior in window.MktoForms2BehaviorsRunCache) {
    return;
  } else {
    window.MktoForms2BehaviorsRunCache[thisBehavior] = new Date();
  }
  // do your stuff here
  
})();
</script>

Once you wrap the code like this it'll work reliably.

I do greatly prefer to keep behaviors in external files, however, since they can be updated without reapproval or touching the Form Editor at all.