You cannot ask Bet Angel to place an SP bet directly.
However, you can get around this by placing a bet 'outside of the money' and then set unmatched bets to 'Take SP'.
For example, if you want to place a BACK bet at SP then place the bet at 1000. The bet will remain unmatched unless the market actually hits 1000. Similarly, if you wish to place a LAY bet at SP then place the bet at 1.01. This will remain unmatched unless the market actually reaches 1.01.
Then select 'Take SP ALL' just before the start of the event and your bets will be matched at the Betfair SP.