The system will only accept Dutch addresses. You should contact customer service directly to have them enter your Belgian address. Usually, this is most easily done by phone, as they are available 24/7 at 030-751 51 55. You may also try to contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Note that it can take a few days to get a reply via social media.

Then the next hurdle will pretty soon come if you also even want to get a subscription/season for NS/dutch railways-it will also not accept non-NL IBAN, though it seems after many yrs they have now even tried to fix this-with limited success. IN that case try to fix it all in 1 in the first call.

Also note that about any season has to be loaded on a chipcard (very similar to the BE-MoBiB), but these oin NL have to be made centrally (not as the MoBiB at the station) and will not be sent by NS to BE adresses-you have to order it before from OV-chipcard and pay for it.