Your application for a refund of the value-added tax component of an invoice you once raised with what has since emerged as a bad debtor can be made as part of your regular periodic value-added tax return, as soon as you have established with reasonable certainty that the debtor in question is unlikely to settle its bills (which will in any event be the case on expiry of a one-year period of the original deadline for payment).
You may use the first value-added tax return you will be filing in 2020 to reclaim the value-added tax included in invoices that have remained outstanding since before 2019.
Any value-added tax you may thus have had refunded to you will obviously have to be returned to the tax man where the underlying invoice in question is belatedly paid.
Dutch version: Btw dubieuze debiteuren