To teach your dog to bark on command, you have to cause him/her to be frustrated. It feels mean at first, but it worked very well for us. We had to teach our Yorkie to bark so that he would ask to go out. Otherwise, he'd just sit there so quietly no one noticed him.
We simply followed this video from Zak George:
How to Teach your Dog to Speak or Bark When you Ask
My fear was that teaching him to bark would make him a yappy dog, but that's not the case for us. He does vocalize more than he did before, but not much. I think the trick is to never yell or raise your voice to get a dog to *stop* barking because they think you're just joining in the fun!
Hope that helps!