The rate will appear if available; no code is needed. When I looked last week for Bangkok properties in mid February it was there; but the sale may have ended by now - or is not available for your dates.
THB 3,000 ++ is a OK rate for Bangkok Millennium - it can not get much cheaper than that, even when the 40% off flash sale occours. As I recall it, I have seen it as low as THB 2,500-2,600 - but that is it. I suggst to go for it - or find a different/cheaper place to stay.