If you mean the pass cover number: you can request it via this form.

You shouldn't need it, by the way, when using Interrail reservation site.

Thanks! Could you also tell me how to add travellers/trips? Nothing is appearing when I login which means I can't access the interrail reservation site.

Yes, I also saw that later and others too. There seems to be a problem since today.

If you're in a hurry, you can call Belgian railways for a reservation. No extra fees, no pass cover number needed, reservation is sent as a PDF via email.

So helpful thanks! I thought it was just me!