The HtmlField has a DisplayMode property, which you should set to "Write" if you want an editable textbox to appear. You've already figured that out.
Are you manually persisting the value from the HtmlField to your property? If so, it would be almost the same to put that value in the TextBox. The HtmlField client component has a get_value() method. You can use that to get the text and then set it to the TextBox using jQuery:
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