If you call their pay phone number, you don't need a pas cover number. 

The number can be found on the B EUROPE website.

You can call  b-europe and make the reservation. No need of Pass Cover Number.

The phone call is rather expensive and there is a fee of 4 EUR.

https://www.b-europe.com/EN/Contact/Contact-Centre

Or you should be able to make the reservation by phone at SNCF. No need of Pass Cover Number and cheaper than calling b-europe.

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

To get your Pass Cover Number, if you still will need it, you need to contact Customer Support through this form and ask for the Pass Cover Number.

https://eurail.zendesk.com/hc/en-001/requests/new

Customer Support is currently overloaded with requests so you will probably have to be patient to get help. Please let Customer Support know what date your travel will start so that they can prioritise your request correctly

@AnnaB 

The information you provide is not correct! B Europe does not charge reservation fees for interrail.

@Sven 

Ok. So the fee for b-europe is only for orders made online?

Yes, only if you use the online reservation there is a booking fee.

This is right-BUT in general most people report having to wait 20-30 mins to get through to b-rail on the fone=that means the fone cost is often higher as the 4€ supplmt for online. BTW-online (just did it myself) charges indeed also 4€ extra and also wants a valid pass-cover-nr (and they seem to check if its really issued-though they can never know if it has been sold or still lying in some office to sell next week).