If you're talking about tweak enemy AI, I of course know the line, it's next line after select_battle_tactic_aux -1 in scripts.txt. But I'm afraid that does not help you.
If you mean adding trigger into a mission. You should find e.g. 'mst_lead_charge lead_charge' in mission_tempates.txt
Code:
mst_lead_charge lead_charge 65538 8
You_lead_your_men_to_battle.
4 1 4160 0 16 12 0
0 4160 0 16 0 0
4 8320 0 16 12 0
4 8320 0 16 0 0
21
21 is number of triggers you should change it to 22.
Then you should add new trigger. You may do it just on the next line.
But before you need to translate ms trigger representation into txt one, or find some how does it. It's not that difficult but I don't wont to waste my time on this routine.