The scan function of bunq displays the payment reference of a Dutch OLA (giro collection form), so a Belgian variant should also work if you have it on paper. You can of course cut and paste from a digital data.
In practice, there is no check that would take place in the traditional introduction of a structured communication or payment reference if you had a separate field at a Dutch or Belgian bank.
However, it does not matter in terms of structure for the transfer whether it is a separate (input) checked field or just text, the description will simply be sent as text and the addressee must pick it up again as a payment reference or structured communication.
So it is best to scan or cut and paste, or check yourself twice before clicking on send.