EDI - Licence Plate Field - TMS / GPS Tracking

Hi. I am looking to intergrate SAP by XML EDI file into a TMS system for GPS tracking. In order to do this I need to pull through the licence plate of the truck at despatch/order level as we use 3rd party so no fixed trucks. Is there a field in SAP that is for this purpose or am I just look at bastardising free text field?