Did you check the command set to find any commands similar to this one?
If this waveform generator has any means of storing a command or sequence of commands ready for execution, you can use a function like ibtrg (Page 5-82 of NI 488.2 Software Reference Manual) to trigger it when you want it. The phase should be 0 because the instrument would not do anything until the trigger so it starts generating from initial conditions you specify.
I am not familiar with the instrument itself, so you will need to look up the commands but this would be the general format for programming.
I don't know if you are using an Instrument Driver or not. Instrument Drivers usually allow you to configure and control instruments for this type of thing. You can find an Instrument Driver for the 33120A through the Instrument Driver Network, www.ni.com/idnet