Find a control from page in webbrowser control

A web page is actually not a control container, so it doesn't have controls. essentially what exists in a WebBrowser control is the ability to service and present HTTP requests, to put it short plain text, which the actual browser then enterpret to more than that.

What you do have is a DOM Document Object Model - Wikipedia[^] and that's all you'll need.

The WebBrowser is a managed wrapper to an Active X control (COM) and it does proved access to the Document through which you CAN interact with a gotten page to some extend WebBrowser.Document Property (System.Windows.Forms)[^].

Back to the point you should be using JQuery in the browser if you at all can control the requested page for any and all interaction with a HTML Document.
jQuery - Wikipedia[^] you can get a reference to a DOM instance of an object (like control in client) and obtain the value by simply typing

var myTextBox;
myTextBox = $("#nameofdocumentobject");
var theValue;
theValue = myTextBox.val();

Or have a play with it in an editor Tryit Editor v3.3[^]