Elpost Shiplab for Magento / DHL Express Configuration / References
A shipment reference is an identifier that you define by completing the "reference" fields during the shipping process. This reference could be a purchase order (PO) number, a customer number, a company name, a Bill of Lading number, or a phrase that identifies that shipment -- such as "10 crates" or "gift for Mom".
Path: Elpost -> Configuration -> DHL Express -> References
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You can use macro paste in a Reference ID
Example: you want to add order id to Reference ID for this you can fill "Order ID is #order_id#" and it will show "Order ID is 12000048" on the label