I put the dewormer in the mashed feed, they eat it all up
You can get cheap syringes in a lot of chemists, or from your vet. If you have the time, I would fill a syringe with something yummy like apple sauce and give it to the horse as you would a wormer.
If you really need to worm them quickly, it's easiest to get help. Give the wormer as normal, then have one person lift the horse's head and tickle its tongue with the end of the wormer, and the other person rapidly rub down the horse's throat to encourage it to swallow. I'd check to make sure the horse actually swallowed it though, because I've known horses to just hide the dose in its mouth and spit it out after we let it go.