You have to create the association with a command word. Follow your dog around all day and wait for it to fart, and while it does, say the command word. Once it figures out the association, it'll know that when you say the word you want a blast.
It'll help if you feed the dog a lot of cabbage and broccoli and such to increase its production, so you don't have to wait around for ages for your chance to train up the word. But you'll presumably be doing that anyway so your dog will always have a few rounds in the chamber for social occasions.