How to set the persistant cookie to true in asp.net..

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.