If this is for one of the b-europe websites:

The best way to make reservations for French trains is to call SNCF. No need for Pass Cover Number.
Press #85 for English, no booking fees, reservations are sent via e-mail.

You can also book by phone via NS. Also no booking fees.

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