The Tax ID is stored within Entities that can be created and stored within your Trabex profiles.
From the menu, select "Client Data". Then "Entities" from the list.
The field you would use for the Tax ID is called EIN - Exporter's Number.
Once you have created this entity, you would attach it to the shipment.
More information on this can also be found in the manual, available under Documents > User guide from within a shipment.
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