Mobile passes do not have a cover number, this is only supplied on paper passes. To get one you must apply to interrail customer services.

Please read this thread on the best ways to get reservations, note the app is not always recommended to be used for reservations.

Thanks - I managed to reserve the Eurostar online, I think I went via Google rather than the App and wasn’t then asked for Pass Cover Note

Where did you do that online and still get the Interrail discount? 

B-Europe, or via Interrail/eurail itself :)

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Without a pass cover number and as I understood not through Interrail/Eurail either. I am curious.

I think it was the Interrail website - I tried through the App but that didn’t work out

The app itself cant reserve anything it just redirect you to a page where you can reserve. Usually the Eurail Reservation Portal