Check for your Customer ID within a order confirmation mailing from 1&1. Since you have an account with 1&1, you should at least have 1 mailing, which would be the order confirmation mailing or a welcome mailing. You can try searching your inbox for "1&1 Internet".
You can also find your Customer ID on any invoice from 1&1. Your Customer ID is also listed at the top-right of every invoice you receive from 1&1. Invoices are also sent to the e-mail associated with your 1&1 account.
If you are not able to find your Customer ID using the methods above, please Contact Us to have a Customer Care Agent assist you.