Lambda that parses data into attributes?

This tutorial should help you out: https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-dynamo-db.html

Your current code appears to do most of this already - you just need to refactor to use a Lambda handler function and get the request body from the event object:

exports.handler = async (event, context) => {
  ...
  let requestJSON = JSON.parse(event.body);
  ...
}

You should then be able to replace the hardcoded values being added to DynamoDB with the data being sent in the request.