I would use jQuery's attr() and jQuery's trigger():
webctrl.ExecuteJavascript("$(#txtFileNo).attr('value', '12345');");
webctrl.ExecuteJavascript("$(#BTN).trigger('click');");
Since you are targeting a single browser, you could use plain old Javascript for this against the Chromium DOM. But, I find jQuery's trigger() to be much easier to use than the alternative.