I don't know this editor that well, but it appears that it doesn't have a "normal" DOM event model.
Going by this page:
https://ckeditor.com/docs/ckeditor4/latest/guide/dev_savedata.html
it looks like you might need to hook in to the 'change' internal event and propagate that into the DOM with some javascript code. Something maybe like this:
```js
var editor = CKEDITOR.replace( 'editor1' );
// The "change" event is fired whenever a change is made in the editor.
editor.on( 'change', function( evt ) {
// getData() returns CKEditor's HTML content.
htmx.trigger("#editor1-wrapper", "changed");
console.log( 'Total bytes: ' + evt.editor.getData().length );
});
```
where, if I understand the library correctly, you have something like this:
```html
<div id="editor1-wrapper">
<textarea id="editor1">...</textarea>
</div>
```
You might need to do an hx-vals attribute on the wrapper as well, to include the editor. I have to say, this editors API doesn't appear to be very friendly to external tools compared to other rich text editors I have seen...