You cannot anymore, that feature got removed in October 2017 with iTunes 12.7. Before that, it was possible to install IPAs onto your iOS device using iTunes, but only those that were linked to your Apple ID. So a random IPA cannot be installed onto your iPhone at all.
Earlier versions of iTunes had access to the iOS App Store, but you had to be signed into iTunes with your Apple ID, in order to download (and sync) apps onto your iPhone. If you downloaded the IPA from the internet and you haven't downloaded it from the App Store using a version of iTunes earlier than 12.7, there is no way to install this IPA. Apple mostly did this to prevent piracy of paid apps.