Create a situation which make her confuse or irritating.
1. Do some workout like jogging, walking for 1 hour.
2. Show your lab her favourite treats.
3. Keep the treats at some height (on window or wall), which can be visible to her but not reachable.
4. As she is hungry, she tries to get it by herself, then she get irritated and bark at them. At that very instant praise her and give her treats. Practice this for some time and you get a dog which bark on command.