You have it in NT10.
The code goes into the PageFooterTag procedure in the Processes Code embed point.
(If you wizard up a new app it'll go in there.)
if (p_web.GetSessionLoggedIn() and p_web.PageName <> p_web.site.LoginPage)
! parameter 1 is the session time
! parameter 2 is the name of the login page.
! parameter 3 is the id of the <div> in the html.
p_web.Script('startCountDown('& int(p_web.site.SessionExpiryAfterHS/100) &',"'& clip(p_web.site.DefaultPage) &'","countdown");')
end