How to place orders during pre-opening session?


Hi, I am a participant of the CASH Algo Trading competition. I have a question as title. 
I read this article Understand Order Matching about trading mechanism for stock market.   
My understanding is that we can still input orders during pre-opening and after close sessions. 
I tried on the backtest system but always got orders rejected. Any one can tell me how to do so?
Hi Darius, you are right. In market practice, you can actually place "At-auction Order" or "At-auction Limit Order" during POS. 

However, for simplicity, our backtest system is currently designed in a way that market price observed during POS and ACS is for reference, and you can only trade during CTS.