Barking on command.
This trick is broken into three segments.
Segment 1 - Induce your dog's drive toward their love. (Toys, food, Ball etc)
Segment 2 - Induce your dog to bark & reward with their love.
Segment 3 - Introduce the bark or speak a command to indicate barking.
In segment 1, you are required to tie your dog again a door or something strong enough to handle its weight. Bring out his/her favourite toy to induce him/her to bark. At the start, even a soft whine, reward him/her with the toy. Repeat this segment, until your dog bark for the toy.
Segment 2, release your dog off command and induce your dog to bark. If he/she could do that constantly proceed to segment 3. If not repeat segment 1.
Segment 3, now is the tricky part. Induce your dog to bark with the bark/speak command. Reward him/her when they bark on your command.
Conclusion, you must combine all these segments. If your dog barks on your command without you rewarded with the toy, you have achieved this training. Happy training and remember, give your dog lots of love and fun.
Cheerios