RAK7201 event structure (specifically for heartbeat)

Issue: Unable to find documentation about the structure of the event sent by a RAK7201 device

Setup: Followed the instructions for the AWS IoT Core for LoRaWAN

Server: AWS IoT Core

In particular, I would like to know how this AWS Lambda Function would need to change in order to get the heartbeat described here. Can you please point me in the right direction? Thank you.