If you fill out the field
PaymentReference, then the ID follows, that you’ve written as a
PaymentReference, for the payment, if the merchant has chosen Instant Transfers.
PaymentReference is not a required string, as it is completely up to the merchant to decide, the needed ID used to reconcile transactions with the bank.
For example, if the merchant wants to use their FIK-Creditor-ID for transactions, then you simply choose the reference number, which can be the merchant FIK Creditor ID. There are no special requirements for the merchant FIK creditor-ID to be able to use it for MobilePay Invoice.