Your enterprise number will be automatically selected as the primary identifier.
If you want to prioritize a different number such as GLN or DUNS, 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.