Insert another textarea

You can use a custom modal dialog and add as many field as needed. Here is an example:

async function modalText() {

  const form = [
    {name: "Name", id: "name", type: "text"},
    {name: "Description", id: "description", type: "textarea" },
  ];

  const data = {
    name: "Default name"
  };

  const modal = await DayPilot.Modal.form(form, data);
  if (modal.canceled) {
     return null;
  }
  return modal.result;
}

You can also build the modal dialog using this online tool:
https://modal.daypilot.org/builder

See also:
https://modal.daypilot.org/documentation/