How to show Order Number/SO Ref on Invoice emails

When creating a sales order or invoice we have an option to input an order number or ref# (see attached images).

For the Sales Order email template we can add a placeholder for the SO_REF. However, for the Invoice email template there's no option to reference either the SO_REF or even the Invoice order number.

Does anyone know of a workaround to reference either of these on the invoice emails?