You don't set a "persitence" property, you set an expiration date/time:
0) Create the cookie
1) Add the user's credentials
2) Set the expire time
3) Write the cookie to the client machine
HttpCookie myCookie = new HttpCookie("myCookie");
myCookie.Values.Add("userid", objUser.id.ToString());
myCookie.Expires = DateTime.Now.AddDays(5);
Response.Cookies.Add(myCookie);
That's all there is to it.