change textarea value doesn't work

[admin]

So, what i'm trying to do is adding a Javascript snippet in wordpress admin new post page, so that when i click a button it fills up the CONTENT textarea where the body of the post goes. Adding it is not a problem, i've already done this, but for some reason filling up the content field doesn't work.

The textarea from the wordpress admin page is:

<textarea rows='10' class='theEditor' cols='40' name='content' tabindex='2' id='content'></textarea>

Also i tried to fill up the POST_TITLE input and it works fine. I know the problem might come from the JS scripts of wordpress for the content field, because it's not a simple textarea, it's a JS text editor. Any suggestions?
And the JS snippet is:

<script type="text/javascript">

    $("form").submit(function(){
        $("input[name$='post_title']").val("a letter");
        document.getElementById('content').innerHTML = "testing";
        return false;
    });

</script>

PS: jquery library already included