events-mf2.js

function App({ onNewMessage }) {
  const handleSubmit = (e) => {
    e.preventDefault();

    const { target: form } = e;
    const input = form?.elements?.something;

    const customEvent = new CustomEvent('message', { detail: input.value });
    window.dispatchEvent(customEvent)
    form.reset();
  }

  ...
}