If you would like to show a message to the user upon successful registration, click on the "Add a Message" button when you are editing a Package. If a message already exists, the button will read "Edit Message".

Fill in your message and click on the "Add" or "Save" button. This message could be additional instructions for  the user now that the registration is done. If you need to convey more information to the participant, you can do so by attaching a file to the Package and mentioning it in this message.