Return Url Problem

Hi,

I have one form suppose news.aspx and on page load I am checking user identity by

string username;
username = User.Identity.Name;
if (string.IsNullOrEmpty(username))
Response.Redirect("~/login.aspx");

I have to append this url with ReturnUrl=news.aspx&AspxAutoDetectCookieSupport=1

how can i do that. ...