The better place to pour it is in front of the filter, where the water is going back into the tank. The heavy circulation will mix it into the water faster, and if poured slowly, it will dissolve faster this way. If it's being used as medicinal remedy, it won't hurt the fish to pour it into the tank directly, the idea is for them to come in contact with some of it.
Important note... when using salt... especially for medicating, use MARINE SALT, not aquarium salt and surely never table salt.
Fish have issues with too much sodium chloride, or long term exposure to large amounts of it. Standard aquarium salt contains too much sodium chloride and not enough good stuff to be useful to the fish, and I don't suggest using it at all. Marine salt is much safer and healthier for the animals.