Is Morton salt substitute KCl? If so, on Chuck's calculator you choose KCl and then the option "add dry compound directly to tank".
For example, 1 tsp added directly to 55g of water will raise K by 15.12ppm.
By the way, I wouldn't input 55g but closer to your real volume of water. I use 85% of the rated volume to take into account substrate, plants and fish, etc.