Hi lola_d,
Apologies for the delayed response. I agree, having that PO number would make the invoice leasier to track. Yes, if you want that reference to appear in the email text you send from MYOB Business, you can set this up under Sales settings > Email defaults.
So if you enter the order/reference number into the Customer PO number field on the invoice, you can include it in the outgoing email message as well.
You can check the steps here: https://www.myob.com/au/support/myob-business/business-settings/forms/sending-emails/set-up-your-default-email-messages?productview=Browser
Hope that helps.
Regards,
Genreve