Unfortunately you can’t do this on the Play+ along with setting a note length on the audio/sample tracks, unlike the synth/midi tracks, samples are triggered per step, you can only add a long sample and use the decay setting to control the length of this, it’s one of the drawbacks of the sample tracks on the Play devices I’m afraid.
There is a wish for a sample based synth engine which should allow you to use samples in this way.
Hopefully we see this in the near future, in the mean time you can vote for it.