The sync_order_details method can be used with either a GET or a POST. If you include the build sequence ID in the URL, it will update the build sequence. If the ID is omitted, it will create a new build sequence.
You can include a cycle_time parameter in the API call. If you include the cycle_time parameter, it will use the supplied cycle_time to calculate scheduled_end_time. If you do not include a cycle_time parameter, the API method will look it up from the Line Cycle Times.
FYI - L2L plans to enhance the /buildsequence/sync_order_details/ method to also accept the schedule_end_date property as well.