You add dechlor direct to the tank. But you do end up using more dechlor because instead of dosing for the amount of water you're adding, you have to dose for the full tank volume. So for a 240l tank you have to dose enough for 240l instead of say 40l (if you did a water change of 4 x 10l buckets). With little water changes it means you're dosing lots more, but with large changes of like 90% it works out near as the same amount of dechlor, and it's a whole lot easier on your back LOL.
I put a hose connector on the end of my gravel cleaner tubing so I can connect it straight to the hose, and then siphon the water straight out into the front garden. Sometimes the kids take the hose and water the plants, sometimes the water just goes down the path and out onto the pavement!
It does depend on what dechlor you use - I use Prime which is much easier to dose to the full tank - I use 2 capfuls which is about 1.5x dosing. If I were trying to dose it for individual buckets I'd end up overdosing way more than that because it's so hard to dose such a small amount.