Hi the way I taught my boy was to use youtube dog barking sounds and treated him when he barked. Then I added the command, then I did without the noise.
For a little bit when I told him to bark, he would bark at me. But I wanted to get him to bark at someone or some things so I used his favorite toy. I would throw it and give him the command and if he barked at it I would release him. I also taught him a "be alert command" and ill say it and give him the bark command as well and he'll stand up tall, look around and bark.
Good luck in finding what triggers her bark!