[Outlook ItemSend Add-in] Modify the email content

In ItemSend Event handler, I want to change the email content. Send back the modified content to the user for confirmation.

function validateBody(event) {
  mailboxItem.body.getAsync(
    "html",
    { asyncContext: event },
    checkBodyOnlyOnSendCallBack,
  );
}

function checkBodyOnlyOnSendCallBack(asyncResult) {
  ...
  if (checkBody) {
    // Modify content in asyncResult.value, which is immutable here

    // Inplace modification or open a new email form

    // Block send if blocked words were found
    asyncResult.asyncContext.completed({ allowEvent: false });
  } else {
    // Allow send.
    asyncResult.asyncContext.completed({ allowEvent: true });
  }
}