You can edit the message that appears in the heading of the "passed" message by clicking following the below steps.
1. Navigate from the home menu to Authoring.
2. Go to either New Message or Saved Messages.
3. Scroll to the advanced message settings.
4. Click Edit Message.
5. Click HTML or Text to update the verbiage that will appear in the header of the forwarded message.
6. When you are finished editing the text, click Update Pass-Along Text.
The pass-along text will then display in your message: