Public Class Form1
  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    WebBrowser1.Navigate("https://login.yahoo.co.jp/")
    While WebBrowser1.IsBusy
    End While
  End Sub
  Private Sub WebBrowser1_DocumentCompleted(ByVal sender As System.Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted
    Dim doc As HtmlDocument
    doc = WebBrowser1.Document
    Dim col As HtmlElementCollection
    col = doc.GetElementsByTagName("input")
    For i = 0 To col.Count - 1
      If col.Item(i).Name = "login" Then
        col.Item(i).SetAttribute("value", "xxx")
      End If
      If col.Item(i).Name = "passwd" Then
        col.Item(i).SetAttribute("value", "xxx")
      End If
    Next
    Dim formElements As HtmlElementCollection = doc.GetElementsByTagName("form")
    For Each form As HtmlElement In formElements
      form.InvokeMember("submit")
    Next
  End Sub
End Class