I have a quart container from Chinese food (actually LOTS f them!). I put the salt in the cup and hold it under the inflow. The salt swirls around in the cup - dissolving. The salt-laden water flows out the top and the undissolved crystals swirl at the bottom. This will dissolve a large amount in a short time.
I also add the salt into the bottom of my bucket I use to refill the tank. EAch time I dump in a new round of water to pour into the tank, more salt dissolves. It is usually completely gone by the time I am done refilling.
I also use that quart container with a lid on and SHAKE SHAKE SHAKE. It is good exercise.
I would be REALLY careful about getting that salt in such a concentration in your filter. I know you put it in front of the filter pad, but I would worry that you are still exposing your media to a much higher concentration of salt than you wish. This can potentially cause problems with your BB.....
Salt is great. Salt is cheap. Salt is a pain at times......