If you don't have either a VAT or KVK number you can use a GLN or DUNS number.
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.
Your VAT number will be automatically selected as the primary identifier.
If you want to prioritize a different number such as KVK or OIN, you first have to add those numbers to your customer file.
As soon as you have more than one identifier added, you will see an extra e-invoicing tab appear in the customer file. Click it to see a list of identifiers for that customer. Select the bullet for the number you want to set as priority and confirm.