Reservation on Eurostar with mobile pass

Hello,

I’m trying to make a seat reservation on eurostar form Paris to London on the b-europe.com site. It is asking me for my pass cover number which I don’t have because I have a mobile pass, rather than a paper one (however I don’t see anywhere that you can’t travel on eurostar with a mobile pass?). Is it possible to get a pass cover number with a mobile pass?

Thanks!