Orders made outside market hours and extended hours trading are queued and fulfilled either at or near the beginning of extended hours trading or at or near the market open, according to your instructions. These orders may be named as off market orders or overnight order.
That is the same as how you would place ordinary orders, except that the orders states are in the "waiting for submission".
Investors can place an order after the end of the relevant period of the trading day. The order is saved by Futu's trading system and automatically submitted to the exchange after the market opens.