otomatic set text and push button

I want to ask one question about ASP.NET htmlparser.

I want to go one url and i want to find textboxes and buton .

After that i will set a value in this textbox and i will push the button.But i want to work this otomatically.I used agilepackage and i went to adress otomatically,i wrote  XPATH and  i found what i want to textbox and i set in my code this XPATH.

var node =document.DocumentNode.SelectSingleNode("/html/body/div/div/div/article/form/input");

But this node is null ...I want to text website's textbox element.

How can i take text box. I will set textbox  like down code.

node.SetAttributeValue("lookupTarget","blabla");

After that i will buton XPATH adress like node and i will RaiseEvent("onclick")

HtmlElement button = doc.GetElementById("btnbuton");
button.RaiseEvent("onclick");

Thanks for help.