The system currently does not have a specific field for that but there is a way to do it:
- Go to the project for the client.
- Put in a note to record the referral source.
- Go to Reports in your top navigation bar, then the Entry & User tab. Click Generate for the Client Project Entries report. Make sure there's a check only for "Show Project Note Entries."
- Export the report to excel.
- Sort through the notes according to "Entry Details/Description."
With the Enterprise Add-on, you can add any custom field to your systems. Once this is enabled, you can look into the article: Adding Custom Fields.