On some window air conditioners (ex. ESA410N and ESA412N), there was a small lever on the louvered grille at the top front of the air conditioner. This allowed a vent to be opened, allowing the unit to pull in fresh outdoor air rather than air conditioning the room.
If you are facing the air conditioner, the lever was on the bottom right side of the louvers. Move the lever to the left for Open and to the right for Closed.
Current models do not offer this feature.