Though you can use the simple entity insert to create a departure like this:
&p15_tripid=[tripname or trip GUID}
&p15_paymentscheduleid=[payment schedule name or GUID]
&transactioncurrencyid=[currency name or GUID]

You can also choose to create a departure as a copy of another departure like this:
&p15_tripdeparturesid=[departure name or GUID]

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.