How to prevent external login using url?

protected void Page_Load(object sender, EventArgs e)
   {
       if (!IsPostBack)
       {
           Response.Cache.SetCacheability(HttpCacheability.NoCache);
           Response.Cache.SetExpires(DateTime.Now.AddSeconds(-1));
           Response.Cache.SetNoStore();
           Response.Buffer = true;
           Response.ExpiresAbsolute = DateTime.Now.AddDays(-1d);
           Response.Expires = -1500;
           Response.CacheControl = "";
           if (Session["username"] == null)
           {
               Response.Redirect("../Login.aspx");
           }
       }