Why you require it in query string??
Simply after login put it in session
like
Session["username"]=txtusername.Text;
and in any page where you want to retrieve use
string username=Session["username"].ToString();
or if you still want to pass as query string
then after login redirect like
Response.Redirect("Page.aspx?uname="+txtusername.Text);
and on Page load of Page.aspx.cs
user
string username=Request.QueryString["uname"].ToString();