Your investment account does not have a unique account number. The username of your investment account serves as your unique account identifier.
Although your investment account does not have a unique account number, you may have already opened a personal cash account to hold non-invested funds. This cash account does have a unique account number (IBAN) which can be seen in the cash account reports in your account. You can see if this is applicable to you under Activity > Reports > Cash account reports. You can also check under Profile > Online forms if you have already opened a cash account.