You are not charged the day-use admission fee at New Hampshire state parks if the VA has given you any percentage of service-connected disability. Extra charges for ski lifts, food service, campgrounds and similar still apply.
There is nothing to apply for. Show your New Hampshire Disabled Veteran license plates or your VA letter certifying a service-connected disability at the park entrance.
New Hampshire Disabled Veteran license plates, or a letter from the VA certifying that you have a service-connected disability.