Is your system sealed or a traditional vented system ie does it have a small tank in the loft?
Either way you would need to drain off some water. If you have the small tank in the loft, switch off the boiler, isolate the water supply to this tank, attach a hose and drain enough water off from the drain valve you mention to just empty the tank. Put your inhibitor into the tank and then reinstate the water supply allowing the tank to refill. Bleed all the radiators, downstairs ones first, and then restart the boiler.
If your sytem is sealed ie you have a small pressure gauge on the front of your boiler, switch off the boiler, attach hose to drain and let out enough water to allow you to put the inhibitor into an upstairs radiator. You will need to remove the plug at the top of the rad opposite the bleed screw and open the bleed screw as you put inhibitor in at the other end. Replace the plug (ensure seal is intact first) close drain off, remove hose, refill system to 1.5 bar on the gauge and then bleed all your rads, you may neeed to add more water to do this. When all rads are bled ensure boiler pressure is at 1 bar and then restart boiler.