Hi,
If you are redirecting from first page to second page you can use QuesryString to maintain the state.
Response.
Redirect("SecondPage.aspx?UserName="+txtUserName.text);
Get values in second page as below
textbox3.Text = Request.QueryString["UserName"];
OR
If you want to maintain or access the username more than one page use session state as below
Session["UserName"] = txtUsername.Text;
Get session value
textbox3.Text = Session["UserName"].toString();