My fiance's horse is a rather picky eater. And he's on joint supplement. What we did was we just gave him applesauce first, so he would get a taste for it and want more. Then we would mix in whatever supplement. Sometimes he won't eat it, so I put it in his feed bowl where he normally licks it (if there's no food in there) He'll have that and then have some of the applesauce. If he's still picky about it, I put some more grain in his bowl and put a spoonful on top of his grain. If that doesn't work, i'll add a handful of grain into the applesauce, so it's kinda chucky, and he'll eat that right up.
Another thing we do sometimes is dip a carrot into the supplement and give it to him.