Belive it or not you made total sense.
It if fine to do your first mixing of saltwater in the tank. But from here on out you need to use a separate container for mixing the saltwater. The reason its ok for the first mixing is because you will not have any fish or corals in the tank at this stage.
The main reason you don't want to add salt directly to an established tank is that the disolving salt both uses oxygen in the mixing process and also can burn soft tissue should the undisolved salt land on it.