Starting a ReadWaveform on an external trigger?

I have a DigitalMultiChannelReader that I need to start reading a waveform when an external line changes.  For example, I want to read Port3 lines 3 and 4 when PFI5 goes high, for a certain number of samples.

          readWaveformTask = new NationalInstruments.DAQmx.Task();
          readWaveformTask.DIChannels.CreateChannel("PXI1Slot6/Port3/Line3, PXI1Slot6/Port3/Line4", "", ChannelLineGrouping.OneChannelForEachLine);
          readWaveformTask.Timing.ConfigureSampleClock(
            "",
            samplesPerSecond,
            SampleClockActiveEdge.Rising,
            SampleQuantityMode.FiniteSamples,
            samplesToCollect);
          readWaveformTask.Stream.Timeout = 1000;
          readWaveformTask.Triggers.StartTrigger.ConfigureDigitalEdgeTrigger("PXI1Slot6/PFI5", DigitalEdgeStartTriggerEdge.Rising);

          DigitalMultiChannelReader reader = new DigitalMultiChannelReader(readWaveformTask.Stream);
          //TODO: How do I make the reader not start the task here?
          reader.ReadWaveform(samplesToCollect);

 The code above will start the task and start reading the waveform as soon as "reader.ReadWaveform(samplesToCollect)" is called.

How can I trigger a waveform aquisition using an external trigger?