A heartbeat sample is already inside Montage. Waveform #2619. The GM set would already have these Waveforms presented as single-Part Performances ready for you to include as Parts merged into your Performance. In [CATEGORY SEARCH] use "Sound FX" as the main category filter and look for "Heart" using the magnifying glass.
Another option is to add the Drum Kit "Electric Kit" as a Part and press the MIDI-note-G3 piano key.