If there's a mod that asks you to edit the Fallout4Prefs.ini (I don't think I've seen a mod that requires me to yet), it should come with a pretty straight forward instruction on how to do so and what to edit. If it mentions a field and no category, you can assume that it's already there and you can just ctrl+f to find it. Something like Tactical Animations's installation instruction on what to edit is basically what you can expect. And of course, make sure you backup the .ini before editing.