I'm working on a TypeScript Azure Function that has an Azure Service bus topic as its output. I can send messages with it without any problem, but I cannot set any metadata as custom property on the message.

I've tried using an object with the same interface as the ServiceBusMessage from the Service Bus Javascript SDK, like this:

import { AzureFunction, Context, HttpRequest } from "@azure/functions";

const httpTrigger: AzureFunction = async function (
  context: Context,
  req: HttpRequest
): Promise<void> {
  const message = {
    body: "my message content",
    applicationProperties: { key: "value" },
  };

  context.bindings.myTopic = message;
};

export default httpTrigger;

But the message is sent as-is, and the applicationProperties is not taken into account. I cannot see them on the Azure Portal in the Service Bus Explorer. The content of the message will be the JSON version of the message object.

I've tried with the extension bundles 3 and 4, without success.

I'm using this function.json file:

{
  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": ["get", "post"]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    },
    {
      "name": "myTopic",
      "type": "serviceBus",
      "queueName": "myTopic",
      "connection": "SERVICE_BUS_CONNECTION_STRING",
      "direction": "out"
    }
  ],
  "scriptFile": "../dist/servicebus-writer/index.js"
}

And this host.json file:

{
  "version": "2.0",
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      }
    }
  },
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[3.3.0, 4.0.0)"
  }
}

How can I set those custom properties?