If you don't have a VAT or KVK number you can use an enterprise number such as GLN or DUNS.
Enterprise numbers in Belgium are published and can be found in the CBE.
Every business has automatically been awarded a DUNS number. If you don't know that number for your company or your customer's, you can look it up in this database.
You can also request a GLN number with your local GS1 organization. Contact them for more info.