Trigger to activate secondary attack?

I've been trying to duplicate how the Siege attack of the Chimera is activated. First, I've been looking for a trigger to change the active attacks (None, #1, #2, Both), but I'm starting to think it's not possible.

Second, I've noticed that the Chimera Lightning Attack is actually their Attack #2. Which leads me think that what the upgrade actually does is to activate Attack #1. Problem is the WE doesn't appear to support making Attack #2 active instead of Attack #1.

Any suggestions, or any of you tried this before?

Thanks,
Trick

Make the custom unit under the Night Elves (base it off a Night Elf unit).

Give it the "Corrosive Breath" upgrade.

Give it the "Corrosive Breath" ability.

Use the trigger to set the "Corrosive Breath" ability to learned.

"Corrosive Breath" upgrade enables the second attack of any unit.

Use the Gyrocopter Bomb upgrade instead, it adds the second attack. Make sure you give your unit the gyrocopter bomb ability and make it able to use the gyrocopter bomb upgrade. Actually, I'm not even sure if you need to add the ability in there, it may give it the attack anyway because you made it able to use the upgrade. Play around with it.