A payment on account for a customer can be added on the sales menu, however, you are currently unable to add this prior to your first reporting period.
This guide demonstrates how to add a temporary reporting period which will enable you to add a payment on account as an opening balance.
Navigate to the Settings > Accounting > Periods menu.
Add a start and end dates to include the date of the payment on account. Click on the Add button.
Navigate to Sales > Unallocated receipt.
Fill in the details of the payment form and hit the Confirm button.
Navigate back to Settings > Accounting > Periods menu and delete the reporting period that you created in Step 1.
Navigate to Money > Opening balances > Cash.
Adjust the opening bank balance to reduce the balance if appropriate and hit the Update button.