You can check the entered user name and password with web.config file and redirect the next page
protected void LoginButton_Click(object sender, EventArgs e)
{
System.Web.Configuration.FormsAuthenticationUser formsAuthenticationUser = new System.Web.Configuration.FormsAuthenticationUser("mahesh", "mahesh");
if(LoginUser.UserName.Equals(formsAuthenticationUser.Name) && LoginUser.Password.Equals(formsAuthenticationUser.Password))
{
Response.Redirect("~/Account/Register.aspx");
}
}