Knowing where patients come from is the cheapest marketing data you'll ever get. Keep the list short and real, and ask at the first visit.
Set up the list
A referral source is one answer to how did you hear about us. Open Settings, then Referral Sources; a new clinic starts with Google, Walk-in, Patient Referral and Social Media. Add a source with a Name and save. Keep the list to things you'd act on: GP referral, Google, Word of mouth, Instagram, Health fund directory. A list of thirty sources produces a report nobody reads.
Archive a source you stop using. Patients who have it keep it, and the report still shows it for the months it was in use; it isn't offered for new patients. A patient can't be saved with an archived source: Selected referral source is archived or no longer exists.
This list is one for the whole clinic; every location uses it, and there's nothing to set up again when you add a location.
Set it on the patient, and read the report
Choose the source under Referral source on the patient's Billing section, at their first visit; it's a one-time answer, so it's rarely worth going back to fill in for old patients. For a patient referred by a particular doctor, record the doctor under Referring doctor on their Details as well, chosen from your contacts. The New patients by source report counts new patients by this field for any period, which is the only reason to keep it filled in.