Integration with wpnotif

Hi, i’m trying to integrate AST with wpnotif, but i dont know what the placeholder for the shipment tracking url is, or how i can create one.
On the wpnotif support page there’s this link: https://help.unitedover.com/wpnotif/kb/how-to-create-custom-placeholders/

Could you please help me understant what i need to do in order to have a placeholder for the shipment tracking url?