NOTE: Starting with the 2019 Q2 Release, we recommend using HTTP POST method for all calls.  This allows you to send parameters including the token in the payload of the message which increases security.  

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.