If you have a mobile pass you need to contact Custom Support. You can use this form to contact them 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 wait for some days to get help. Please let Customer Support know when you plan to start travelling so that they prioritise your request correctly. 

Thanks for reply.