C/H addatives/concentrate

I have a wood burning stove with back boiler. Its quite difficult to access the loft/feeder tank, so I would like to use the Screwfix Concentrate(No nonences) additive. This comes in a sealant like tube, with a small kit. is it just a case of attaching this to the stopcock on the bottom of the boiler, opening and filling. Does the water go everywhere? How much to use? does it block drain for future?. ANY help advice/tips would be appreciated.? Thanks
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