When registering for Peppol we need your Peppol Participant ID (aka Peppol-Participant-Identifier aka Peppol-Endpoint-ID). This ID has the following format:

4-digit schema code, followed by a colon, followed by the value
eg. 0088:9110019474691

On the following page you can find all schema codes and what they mean:
https://docs.peppol.eu/poacc/billing/3.0/codelist/eas/

Meaning if you are from the Netherlands, you need to supply 9944 as the Schema Code, followed by a colon, followed by your VAT Number.

When in doubt, always ask your Tax Advisor.