Here is a method I have used as a Windows Forms Application...
On a form add a WebBrowser control and a button
Behind the button .Navigate to your chosen web site
Convert the WebBrowser's .Document to a HtmlDocument and you can then access the elements in the page
For example this snippet selects a value on a dropdown list containing Area codes and then clicks the submit button
HtmlDocument md ;
md = (HtmlDocument)webBrowser1.Document;
md.GetElementById("AreaID").SetAttribute("value", "109");
md.GetElementById("cmdSubmit").InvokeMember("click");
This won't necessarily work for all webforms and you will need to work out what the element Ids are
Here is an article that uses a similar method
http://scyanide.com/2009/07/use-c-sharp-to-fill-out-web-forms/[
^] there are probably many more