Play, jump around and wrestle with your dog. This gets your pet in an energetic mood.
Once your pet is full of energy, show him a treat. Hold the treat where your dog can easily see and smell it. Hide the treat, and then bring it out again. Continue acting excited.
After a while, your energy and refusing to give the treat causes your dog to bark. Offer the treat and praise when he barks.
Once your dog is barking in response to your actions, add in the cue word. Try to say "speak" before each bark. Continue to reward your pet after each bark.
You may add in a hand signal to go along with your verbal cue. Dogs understand physical cues better than verbal ones. If you have a marker, such as a dog clicker, use this after each bark to reinforce the behavior.
It may take a few days before your dog catches on. Keep practicing, and your pet will learn.