Cynthia usually adds about 3 extra tablespoons and uses Silk brand Original Soy Milk in this recipe. If you don't have Silk available, or want to go soy-free, you can use any type of original or vanilla milk alternative (almond, rice, etc.). Keep in mind, the regular Bisquick is not soy-free, but the Heart Healthy Bisquick (at last ingredient check) was soy-free.
Variations: For gluten-free, try the Bisquick Gluten-Free. It should work in equal amounts in this recipe, since it does contain a binder.