Most banks do not require your bank account number on the ACH file as they prefer an unbalanced file. When you setup ACH service with your bank, the bank will setup ACH service for a specific bank account, and the file will be transmitted (in the bank’s portal) to/from that specific account.
However, if your bank is requesting an offset or a balanced file, you can enable this by clicking on the Settings tab > ACH File Setup > and navigating to screen 4. Once here, you will need to check to enable the offset, enter your bank account number, ABA routing and company name. You do not need to enter an offset ID.