how to access the login controls login button to change the default postback url.

You can use the below to custom validate the user on click of Login button in Login Control and redirect as per your requirement -

protected void LoginUser_Authenticate(object sender, AuthenticateEventArgs e)
        {
            int userID = 0;
            Login login = sender as Login;

            //The below line validates the user from DB
            userID = controller.Authenticate(login.UserName, login.Password);

            e.Authenticated = userID > 0 ? true : false;
            if (e.Authenticated)
            {
                Response.Redirect("Admin/Main.aspx");
            }
        }