What model water meter do you have? I have that exact meter and I wouldn’t be able to improve on the description in your link as to where the prpobe goes. Look for a black plastic cap over the hole.
Heads up though… using the official Elster T-probe (mechanical reed switch) I found there are situations where it becomes unstable. It seems to depend just where the magnet comes to rest after a water draw. If you’re unlucky and it rests in the critical spot, you can get spurious triggers. This isn’t the standard mechanical switch de-bounce issue (which you also need to deal with). This will give random pulses all night long if the magnet comes to rest in just the right spot. I ended going with an electronic version which has been rock solid.