When placing bets before start time, that is before market turns in play, it is possible to set your strategies to use "Staking->Bets persistence" rule and to set "Keep" bet persistence. This setting will tell Betfair to keep your Unmatched bets alive when market turns in play.
By default Betfair cancels all Unmatched bets when market turns in play, but if you set Keep persistence then your bets will stay alive.
Bets persistence also allows you to set "Take SP" (Start Price) persistence and to place Start Price (Market On Closed or Limit On Close) bets, but this is only available for bets placed before in play. "Take SP" will tell Betfair to match your unmatched bets at Betfair Start Price and this will work only for BSP markets and unmatched bets above Betfair minimum bet size.