I was wondering if I have to add aquarium salt to the aquarium every time I do a water change, and if I add it to the water I'm about to add to the aquarium or add it to the aquarium after I added the new water?

I would not add aquarium salt to your tank unless it is needed for treatment of disease, high nitrites when cycling, etc. Goldfish are freshwater fish and salt is not needed on a routine basis.

If at some point you need to use salt for one reason or another, you dissolve the salt in water before you add it to the aquarium.