If the object of the exercise is to provide heat in the bathroom whenever the hot water is on (irrespective of whether the central heating is on or not) then the flow has to come from the primary flow before the hot water cylinder (and radiator circuit) and the return to feed into the primary return as the last connection to the boiler.
If you connect it to the hot water cylinder you'll ruin the radiator very quickly as it will rust. With the pipes fed into the primary the radiator will benefit from the inhibitor in the central heating circuit.
Some people like this system because it means they have a hot radiator in the bathroom during the summer when the central heating is turned off.
If the new radiator is replacing an old one and the new one is the same size then all you need do is turn off the valves at each end, empty the radiator, remove then replace and bleed.