On the left side of the dashboard, go to the "receivables" tab and click on "reports". Then go to "customer status tracking report".
Click on the "Export" button on the top right side of the report.
Choose the type of file you prefer and enter your email to receive the report in your inbox.
Go to your inbox and download the report. In the report, there is a column called "External Ref Number" and this column is equivalent to your "Customer ID".