Actually, I have managed to do this.
First, take your dressing recipe and add it to your favorites on your Nutrition Tracker. I do this by hitting the Recipes tab, adding the item to whatever meal, and ticking the "add to favorites" box. I then delete the item from the meal, but it remains in my favorites.
Next, when you are in the Recipe Calculator for your potato salad, go in to "add an ingredient" and all of your favorites (including the dressing) will be in the drop-down menu. All of the nutrition info will be added with it.
Hope this helps!