I cant get async-await to work when using postMessage and a MessageChannel

const iframe = document.querySelector("iframe");

const sendMsgOnPort = async (msg) => {
  const channel = new MessageChannel();

  const testfunc = async () => {
    channel.port1.onmessage = ({ data }) => {
      channel.port1.close();
      if (data.error) {
        return data.error;
      } else {
        return data.result;
      }
    };
  };
  iframe.contentWindow.postMessage(`${msg}`, "*", [channel.port2]);
  await testfunc();
};

iframe.addEventListener(
  "load",
  async () => {
    console.log(await sendMsgOnPort("msg"));
  },
  true
);

and the child I have

window.addEventListener("message", (event) => {
  try {
    event.ports[0].postMessage({ result: `${event.data} back` });
  } catch (e) {
    event.ports[0].postMessage({ error: e });
  }
});

I can get it to work with

const sendMsgOnPort = (msg) =>
  new Promise((res, rej) => {
    const channel = new MessageChannel();
    channel.port1.onmessage = ({ data }) => {
      channel.port1.close();
      if (data.error) {
        rej(data.error);
      } else {
        res(data.result);
      }
    };

    iframe.contentWindow.postMessage(`${msg}`, "*", [channel.port2]);
  });

but is there a way to do this without the new Promise((res, rej) => {