How to get the data sent by device?

I figured out how to set up the payload codec option to encode and decode the data. Just for posterity, I'll leave the code here:

/**
 * Decode uplink function
 * 
 * @param {object} input
 * @param {number[]} input.bytes Byte array containing the uplink payload, e.g. [255, 230, 255, 0]
 * @param {number} input.fPort Uplink fPort.
 * @param {Record<string, string>} input.variables Object containing the configured device variables.
 * 
 * @returns {{data: object, errors: string[], warnings: string[]}}
 * An object containing:
 * - data: Object representing the decoded payload.
 * - errors: An array of errors (optional).
 * - warnings: An array of warnings (optional).
 */
function decodeUplink(input) {
	if(input.bytes.length == 0)
		return { data: {}};
	var result = "";
	for (var i = 0; i < input.bytes.length; i++)
		result += (String.fromCharCode(input.bytes[i]));
	result = result.replace(/\0/g, '');
	return { data: JSON.parse(result.trim())};
}

/**
 * Encode downlink function.
 * 
 * @param {object} input
 * @param {object} input.data Object representing the payload that must be encoded.
 * @param {Record<string, string>} input.variables Object containing the configured device variables.
 * 
 * @returns {{bytes: number[], fPort: number, errors: string[], warnings: string[]}}
 * An object containing:
 * - bytes: Byte array containing the downlink payload.
 * - fPort: The downlink LoRaWAN fPort.
 * - errors: An array of errors (optional).
 * - warnings: An array of warnings (optional).
 */

function encodeDownlink(input) {
  var output = JSON.stringify(input.data);
  var array = Array.from(output, (char) => char.charCodeAt(0));
  array.push(0);
  return {
    bytes: array,
  };
}

If you can see the .../event/up message using MQTT, you will see a data key in the JSON payload. Please note that this data key holds the data as a base64 encoded string. You can use this site to decode from various encodings: https://www.asciitohex.com/. In your case it should decode to the test "Hello, world!" if that is what your device is sending as bytes.