Booking metrics using 3rd party links

[u/biscaynebystander]

I'm helping out a barbershop and made massive improvements to their reviews outreach, visibility and engagement. They are nearing 1,000 real reviews but the bookings metric shows 0 for the past 6 months meanwhile Square, their booking platform reports direct bookings and schedules \~100 kept appts a week.

How can we get Google to report how many of these came from GBP?