public Form1()
{
    InitializeComponent();

    this.webBrowser1.Navigate(你的登录网址);
    this.webBrowser1.Navigated += new WebBrowserNavigatedEventHandler(webBrowser1_Navigated);// 注册 Navigated 导航完成后 事件
}

private void webBrowser1_Navigated(object sender, WebBrowserNavigatedEventArgs e)
{
    this.webBrowser1.Navigating += new WebBrowserNavigatingEventHandler(webBrowser1_Navigating); // 第一次导航完成后注册 Navigating 导航开始之前 事件
}

private void webBrowser1_Navigating(object sender, WebBrowserNavigatingEventArgs e)
{
    System.Diagnostics.Process.Start(e.Url.ToString());
    e.Cancel = true;
    this.webBrowser1.Navigated -= new WebBrowserNavigatedEventHandler(webBrowser1_Navigated);
    this.webBrowser1.Navigating -= new WebBrowserNavigatingEventHandler(webBrowser1_Navigating);
}