Write to the LiveChat and provide us the email address you used to sign up for your Royal Mail OBA account - we will need this information to create a separate account for you in order for you to be able to add the EORI code.
You will then get an email from an app called Intersoft which is owned by Royal Mail. You will be asked to log in here: https://multiorders.intelligentshipper.net/Account/Login
Once you log in, go to this area to set up the EORI code: https://multiorders.intelligentshipper.net/Maintenance/CustomerCustoms/VatEoriSettings?noAnimation=Tru