This attribute specifies the Lead Duration of the maneuver - how long before the maneuver starts the maneuver attitude.
Astrogator <Satellite Object Path> SetValue <Attribute Path>.AttitudeLeadDuration <Value> [{Unit}]
Outside the time span from Lead Duration to Trail Duration of the maneuver, the Vehicle Attitude is used unless there are multiple maneuvers in the Mission Control Sequence (MCS). The attitude during the lead time is held inertially fixed as the attitude at ignition. Similarly, the attitude at the maneuver's end is used as the inertial attitude after the maneuver segment. The model transition from the Vehicle Attitude to the Maneuver Attitude is instantaneous as is the transition back to the Vehicle Attitude at the end of the Maneuver Attitude.
Astrogator */Satellite/Satellite1 SetValue MainSequence.SegmentList.Maneuver.FiniteMnvr.AttitudeLeadDuration 500 sec
STK Programming Interface 11.0.1