I have a user control that contains a text box, an HtmlEditorExtender, and a button. The user control is loaded into a parent page using LoadControl().
Literal lit = new Literal();
lit.Mode = LiteralMode.PassThrough;
lit.Text = HttpUtility.HtmlDecode(HTMLTExt);
TextBox1.Text = lit.Text; // The text box which HTMLEditorExtender is attached to