You'll register for INVOICENOW via a certified third-party app/service that integrates with and extends QuickBooks Online's functionality, @Victor.
QuikBooks Online (QBO) Singapore does not offer native, built-in support for the InvoiceNow/Peppol network. Instead, QBO achieves compliance with InvoiceNow through an external Peppol Access Point provider that acts as a bridge.
You can visit the QuickBooks App Center to discover IMDA-accredited InvoiceNow or e-invoicing add-ons, or head to the Singapore-specific QBO website to view recommended partners.
If you have any other questions, let us know in this thread.