Dish soap does not kill armyworms but can be used to detect if you have an armyworm infestation.
The army worm soap test uses a mixture of dish detergent and water, which is then poured on the weakest part of your lawn. The soap irritates the skin of the armyworm causing them to surface, making it easier to see and identify armyworms in your lawn. Here’s a quick video showing how to do the soap test on a small part of your lawn: