Since Bookmaker uses DGS you are actually better off by not even mentioning the parlay right away since chances are the clerk will be easily confused, you should start out by giving the clerk the pick you want to use to fill in the parlay and then tell them to add that team to your open parlay that you already have pending.
If you have mulitiple open parlays in pending then you will want to identify which one it is by telling them the first team on the parlay and the risk amount of the wager.