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

The phone call is rather expensive.

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

The best way to make reservations for French trains is to call SNCF. No need of Pass Cover Number. 

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.

You can generate a pass cover number for mobile tickets at the following link without having to contact customer service:

https://www.eurail.com/en/book-reservations#/generatePassCoverNumber?utm_source=chatbot&utm_campaign=pcn
You can access this also by using the chat bot function on the help page by typing pass cover number.