Lay bets being matched when I don't want them to be

Hi,
if I have understood well, you want to lay drifters, only when they have drifted (at 150/1 for example), but you want the bet is placed before this happens, right?
This is called Limit Order, and it is quite common in Stock Exchange and Forex trading, but it is not present in Betfair markets.
To achieve this, in Betfair markets, you have to wait until the runner has drifted to the odd you wish, and then place the bet (as fast as possible).
Otherwise, you will match the bet at the best lay price available at the moment of the placement: if the runner is at 20/1 and you ask for 150/1, the bet will be matched immediately at 20/1.