You Really intend to duct tape a twitch to a horse to deworm?!?!?
Here's what to do, and it will take a Saturday and Sunday to accomplish:
(1) save a used dewormer cartridge
(2) fill it with applesauce or molasses--you can buy a small amount at any grocery store
(3) halter your horse and hold the lead
(4) use approach and retreat, 5x-7x, stop and pet the horse, until he/she is showing no fear..REPEAT, REPEAT, REPEAT ALL AFTERNOON
BE PATIENT!!!!! According to Dr. Miller (of imprinting foals fame), it will take 20x as long to retrain fear out of a horse as to train a horse who has no fear. Look for your horse to become curious about the syringe.
(5) when you horse lets you get the syringe next to his mouth, squirt the applesauce or molasses on the side of his mouth, then retreat
Take one whole afternoon and desentitize your horse to the syringe. At some point, IF YOU ARE PATIENT, you'll be able to squirt a little sweet stuff into his/her mouth with the syringe.
At that point, stop.
Next day, repeat, until he starts looking for the syringe and the sweet stuff.
When you get him to not fear the syringe, deworm the next time with QUEST, bc it is a dissolvable gel and your horse won't need you to hold his/her head up to not spit it out.
REINFORCE your retraining by using a syringe with sweet stuff in lieu of a treat. Then, he/she will be enjoying deworming, instead of dreading it.