Unless there is a towel warmer or filter where it can be added, I found the easiest way is to turn off the cold water supply to the expansion tank, drain off just enough water from the system so that only the expansion tank itself empties (rest of the system remains full). Then I take out a couple of pints or so extra (you'll have to judge how much your system needs) so that the pipe from the expansion tank to where it joins the heating system pipework is emptied, plus enough volume for the quantity of inhibitor being added. I empty the inhibitor into the expansion tank and watch it disappear, then restore the cold feed to the tank.
When I've done this, as the rest of the system remains full, It's avoided creating air locks and having to bleed all of the radiators afterwards.
I keep a bit of inhibitor back (quarter of a cup) to tip into the header tank as it's filling to treat the water that will be in there.