Where do you want to make a reservation?

If you have a mobile pass you can get your Pass Cover Number here:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Many thanks Anna.  I was trying to book eurostar to Brussels on interrail website but my payment wouldn't go through & all details were correct so I've booked on sncb website which worked fine - cost €2 more tho! 

The SNCB fee is 4 EUR per booking and the Interrail fee is 2 EUR per train and person, so if you book for 2 persons or 2 trains for one person at SNCB the fee is the same. 

I was only booking 1 train for 1 person which cost 4 euro on sncb & would have been 2 euro on interrail.  But I'm more bothered about why my payment didn't go through  - very annoying!