Some people spray the blossoms with apple cider.

It possibly might work to help setting fruit in cold weather but just choose varieties that do it as part of their genetic make up to begin with...
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If you read the FAQ here at GW,linked to below, you'll see that indeed as Lindalana just said, only to be used in cold weather.

That means in areas where the temps are too low toallow for normal pollination such as the PNW/

There are many different formulations of Blossom Set, and different hormones are used. And as said in the FAQ and I'll add a few words of my own, since hormone mediated fruit set is abnormal, fruits can me misformed and also the fruits can lack seeds.

No different really, from parthenocarpic varieties, many of which were bred by Dr. Jim Baggett in Oregon, which also allow for abnormal fruit set with the same problems.

If temps are OK for normal pollination and fruit set, and pollen is not clumped and there's been no sustained high heat or humidity, then look elsewhere for the reason for the blossom drop, also discussed in the link below.

Carolyn

Here is a link that might be useful: GW blossom drop FAQ

Forget the set spray and go buy yourself a $5 battery operated toothbrush. Place the vibrating brush near the tomato flowers and voila, you've duplicated what the wind does. I do this all winter long in my greenhouse to pollinate the tomatoes.