Though you can use the simple entity insert to create a departure like this:
https://data.peak15systems.com/beacon/service.svc/insert/entity/p15_tripdepartures? token=yourtokenhere &p15_tripid=[tripname or trip GUID} &p15_startdate=2019-01-01 &p15_enddate=2019-01-10 &p15_tourdays=10 &p15_minguests=2 &p15_maxguests=2 &p15_paymentscheduleid=[payment schedule name or GUID] &p15_departure_guesttypes=0 &p15_planforleaders=0 &transactioncurrencyid=[currency name or GUID] &statuscode=Quoting
You can also choose to create a departure as a copy of another departure like this:
https://data.peak15systems.com/beacon/service.svc/insert/complex/copyDeparture? token=yourtokenhere &p15_tripdeparturesid=[departure name or GUID] &p15_startdate=2019-01-01 &p15_tourdays=10
Sending in p15_startdate is optional. If you don't send in a start date, the new departure will keep the start date of the original departure.
Sending in p15_tourdays is also optional. If you don't send in tour days, the new departure will keep the same length as the original departure.
When you copy a departure, it copies all the costs and prices over as well.