I use quiessence (Magnesium/chromium) for cresty neck & IRS problems and it did have a settling effect on my mare that had become very high strung but I wouldnt say it would be enough to calm a horse in a situation where it might over react and panic
Hot and young doesnt essentially = spooky
Most TB's I've had have been the least spooky horses I've owned and my very fizzy forward going mare never spooks at anything
I'm not sure what you're expecting your horse to do in the trailer but if its a first time then I'd suggest you just go along the road and back a few times first
I think it mostly helps "take the edge off" from what I've seen. I personally don't like using ANY kind of calming supplement for horses. I only used this for the IR and cresty neck problems with my mare. Unfortunately it had a calming effect.