You must click on the link in the Microsoft Rewards app once it shows that you spent $40 to receive the bonus points. I think the countdown timer is til 2:00am Pacific Time.
It also does not round up to the nearest dollar. Mine showed $30, but my true total was around $30.50 or so. Once I bought a $9.99 game it showed $40.
Guess it went away at 1:00am PT, an hour before sales ended. I bought some stuff and it was already gone...Was gonna get them anyway, but I shouldn't have waited so long.