Yes, putting it into the venturi hole would be best.
When you're dissolving CO2 into a tank directly, for example with an atomizer or ceramic diffuser, you want bubbles to spend as little time as possible in the tank. To do this, you want little bubbles because they A) maximize surface area with respect to volume, leading to quick dissolution and B) they're less buoyant, so they'll float to the top slower and maximize contact time with the water. A bubble that reaches the top is lost CO2. However, in a reactor like this, you want the CO2 bubbles to stay in the reactor. To do this they need to float up faster than they're pushed down by the current. Ideally, a bubble would stay intact in the reactor until it would be small enough to be pushed out, at which point it would leave the reactor. If your bubbles are being broken up by your powerhead, then they'll be on average smaller and more of them will be broken up.
Lets say each bubble is of size '50' when they exit your CO2 line, and they need to be size 5 to escape the reactor. If injected directly into the reactor, they will (ideally) bumble around until enough CO2 has dissolved for them to be size 5, at which point a single size 5 bubble leaves. Now lets say you instead run it through the powerhead where it gets chopped into 5 size 10 bubbles, each of which bumble around the reactor until they are each size 5 and escape. That's 5 bubbles of size 5 that will leave the reactor, for a total wasted CO2 of 25, whereas in the first scenario a single bubble of size 5 escaped, for a total wasted CO2 of 5. Some of the earlier Cerges reactors had issues with bubbles escaping because the way that the CO2 was injected broke up the bubbles in the way described above, which is proof of concept enough for me.
Of course, it gets more complicated than that, with slower flowing reactors having potentially smaller bubbles escaping, and then there's issues with too much CO2 entering a reactor, but this is really a simplified example. Since you have a convenient venturi hookup, you can experiment to see which one gives you the best results.
