Estoy utilizando esto :
Código:
WebBrowser1.Document.GetElementById("email").InnerText = TextBox1.Text
WebBrowser1.Document.GetElementById("input_password").InnerText = TextBox2.Text
WebBrowser1.Document.GetElementById("submit_button").InvokeMember("click")
WebBrowser1.Document.GetElementById("input_password").InnerText = TextBox2.Text
WebBrowser1.Document.GetElementById("submit_button").InvokeMember("click")