There used to be a Waiting for Customer Notice quick action that did what, I think, you are trying to do here. If you don't already have that, then here is what you can do with what you already have:
- Append your quick action and add the "Send Email" action.
- In the body of the Send Email action, insert the field that contains the message.
The other way you can do it, if you didn't want to add the extra field, is to just use the $(Prompt()) function within the message body field of a Send Email quick action.