The Html Editor is one of the unique Ajax Control Toolkit controls, becuase it does not inherit AjaxControlToolkit.ExtenderControlBase
(server side) nor inherit AjaxControlToolkit.BehaviorBase
(client side).
So you can't use $find
javascript method to get access to the behavior instance on the client, It inherits AjaxControlToolkit.ScriptControlBase
(server side) and Sys.UI.Control
(client side).
To get access to control instance on the client, you use the control property on the DOM element it self as follows:
<script type="text/javascript">
//considering the editor is loaded.
var editorControl = $get("<%=editor.ClientID%>").control;
//1. For setting content:
editorContorl.set_content("Sample Content");
//2. For getting content:
var content = editorContorl.get_content();
</script>