A new subfolder would need to be created in the original prospect folder number. This then shows as a dropdown in the Prospect folder box on the Prospect maintenance page. To do this
Click Sales then click Prospect Maintenance.
Click the Prospect Folder then double click the Prospect number.
Create a new folder in the prospect number. Close the box and the prospect maintenance box.
Reopen prospect maintenance. The new number now appears in the prospect folder.
