When the load completes, you get the DocumentCompleted event - or you would if that code compiled, which it won't. You get a "Unrecognized escape sequence" error from the compiler, so no executable is generated. Start by changing it to this:
and see if that helps - it probably won't, unless your file is in the executable directory, but that's your problem, not mine! :laugh:
So even when you fix that, and it finds the file, the load won't happen instantaneously: you need to get your element in the handler, not an unrelated method.
In the handler, you can set the textbox using the SetAttribute method:
HtmlDocument doc = webBrowser1.Document;
doc.GetElementById("myId").SetAttribute("Value", "Text I want in the text box");