If your IBAN is BEXX YYYY XXXX XXXX, then your "account number" is YYY-YXXXXXX-XX.
The "Rekeningnummer" (bad translation of Apple) is the routing number.
This are the first 3 digits of the old account-number format, so YYY.
You'll see after you fill in this, Apple will pre-load your bank below as verification.
So:
IBAN: BEXX735XXXXXXXXX
Account nr: 735-XXXXXXX-XX
"Rekening nr: 735"
That should do the trick :)