Try finding a small nail slightly larger than the hook's wire. Cut the head off the nail and lay the nail in the hook shank slot where the bend of the hook comes away from the head [bottom of that cavity], then close the mold holding the handles tight and lay the mold on a flat, hard surface and give the mold a tap with a hammer right over that cavity. You will feel the mold give a little. This should create enough extra vent to allow all the air our when pouring. You may get a tiny bit of flash along the hook shank but that will clean right off with a knife.
Lay your nail in the shank groove before doing this to see how close the nail is to the mold's cut for the hook. You want a nail that is very close to seated in that cut and when you tap the mold with the hammer you don't want to rap it, just give it a half-firm tap. while holding the handles together tightly so you can feel the mold shift or settle on the nail.