For the record, this is only going to concern those of you who actually plan on partaking in slalom skiing. If you do, the foot you place forward needs to be the one that provides you with the best balance. From user to user, this is going to be different.
And, one of the best ways to determine this is with combo skis. With both skis attached, shift your body weight over to one ski and lift the other ski out of the water. Do this both ways to conclude which foot you feel more comfortable on. This will then be the foot you place forward on your slalom ski.