My dog never barks, but id like her to learn to bark to be let out. How do I get her to?

I've only ever heard her bark 3 times, and its when we had guests over she didnt know, and saw them approach, I've tried the method of teasing her until she barks, but she's super timid bc of her previous owners and just gives up. She gives up on the toy, or treat, or whatever before even thinking about barking. Please help me help her gain her voice back.