Code:
;------------------------------------------
Trigger goodsiegeattacker
WhenToTest PostBattle

Condition WasAttacker
        and IsGeneral
        and WonBattle
        and I_ConflictType Siege

Affects GoodSiegeAttacker 1 Chance 50

This thread (Events, Conditions, and Commands) lists the I_ConflictTypes:

https://forums.totalwar.org/vb/showthread.php?t=43121


Here's the relevant section:
Identifier: I_ConflictType
Trigger requirements:
Parameters: conflict type (SuccessfulAmbush, FailedAmbush, SuccessfulNightRaid, FailedNightRaid, Normal, Siege, SallyBesieger, Naval, Withdraw)
Sample use: I_ConflictType SuccessfulNightRaid
Description: Test the type of conflict (ambush, siege &c.)
Battle or Strat: Either
Class: BATTLE_CONFLICT_TYPE
Implemented: Yes
Author: Lee