In that case the problem does not have a solution. There is no way to accurately tell which bet comes from which source, because it may be split, matched at a different price, voided etc.
Please see the attached example of a trigger that counts the bets placed by a trigger. This, however, does not help if you want to count only the matched part of those bets. If that is the case, then I'm afraid this trigger request cannot be accommodated and you can formulate a different one in this topic.