I think you're barking up the wrong tree. I don't think she needs a calming supplement. I agree with everyone who said to alter her diet, and rule out pain. If it's only when she is being ridden, it's more likely a bad saddle fit than she is just high strung.
After you have ruled all that out, and still think you need a supplement, you can try lavendar oil, and put it around her nostrils. The smell is supposed to be soothing. I'm sure there are more oils out there you can use, but I'm pretty sure this is something you can fix with good training.