get textbox value in c++

I have a dialog window.
In javascript I can get textbox value by
document.GetElementById('f1').value.

in c++ GetNodeValue method of textbox DOM element returns empty string.

Now I use
<textbox id="f1" oninput="this.setAttribute('value', this.value);"/>
then I can get element attribute.

I suppose there is direct way to get the value in c++

GetTextContent, GetUserData return nothing.