If all the air is purged properly from the system, why would you need to add anything anyway? After the water is heated and air purged out, there should not be any dissoolved oxygen in the water to cause any rust. And an auto air vent should get rid of any air from any make up water that gets added in. You asked HOW to add the inhibitor, but I am curious as to why? adding any stuff to the boiler water may somehow get leaked into the domestic water, and that is not a good thing, I believe.