I got very worried when Kodi stopped eating his lunch. I called his breeder and asked what I should do. Her response? "He probably just doesn't need it any more."
I stopped offering lunch and life went on as normal.
I would NOT put PB on her food to entice her to eat, for a number of reasons. It has a lot of fat and a lot of calories, and although it won't hurt her, it also isn't very good nutrition for her. Choose a food that YOU think is a good choice for her and stick with it. Put it down at meal time, and if she hasn't eaten in 15 minutes, pick it up. Don't offer ANYTHING else until her next meal time. Then do the same thing. She might hold out for a meal or two or three, but she WILL start eating. Healthy dogs will NOT starve themselves.
Havanese get a reputation for being fussy eaters because they are very smart and train their people into BELIEVING that they are fussy eaters.
Be strong, and she will come around.
Now, of course, this advice is based on a healthy puppy, in normal, good body condition. If your puppy is under-size, under weight or ill, don't do ANYTHING without consulting your vet first. But the fact of the matter is that there are many too many overweight small dogs, including Havanese. A lean dog is a healthy dog!