How to set the contents of a HtmlEditorExtender from the client?

I have an asp:TextBox with an Ajax HtmlEditorExtender

<asp:textBox ID='reply' ...  runat='server' Text='xxx' /><ajax:HtmlEditorExtender TargetControlID='reply' runat='server />

renders into:

<textarea .... id="MainContent_reply">xxx</textarea><input type='hidden' id='MainContent_HtmlEditor_ClientState /><div Id='MainContent_HtmlEditor_popupDiv'><div Id='MainContent_HtmlEditor_btnCancel' style='...' unselectable='on'>Cancel</div></div>

I don't understand the Hidden control nor the Cancel div.

I tried setting the innerHTML, value, innerText properties of the <textarea> but it didn't work.  How can I set the text inside the rich text editor?

Thanks.