I'm not experienced with this product. However, my engineering sense is that putting it in the expansion tank is sufficient. As the engine cycles between hot and cold, coolant moves into and out of the expansion tank. After some cycles, the liquid in the expansion tank will have mixed well with the coolant in the engine and radiator.
As such, it might take a couple of engine thermal cycles for the additive to be fully effective.
2020 GMC Denali 2500HD Crew 4X4 Gas 6.6L Rockwood 8280WS (30' 5th)
1000W solar; 322AH LFP battery, 900W PSW inverter, NovaKool RFU9000 12V fridge, CPAP, Inverter microwave.