I'm not sure how to give someone else's horse an apple... A few people have given mine apples and I want to do that

When you go to the horse's page, at the bottom there will be a link to treat the horse. However in order for it to show up, you have to have treats in your inventory. If you don't have treats, you can buy them from the game store with store credits in the item section or purchase them from other player's stores with ebs