SHDocVwSample06

test.cs

public void ClickButton()
{
    var IE = new SHDocVw.InternetExplorer();
    IE.Visible = true;
    object URL = "http://www.google.com/";
    IE.Navigate2(ref URL);
    IE.Wait();
    var doc = IE.Document as mshtml.IHTMLDocument3;
    doc.getElementById("lst-ib").innerText = "ぐるぐる";
    IE.Wait();
   //検索ボタンをクリック item(index:) or item(name:) で指定可能
   doc.getElementsByName("btnK").item(index:0).click();
}