Facebook/Instagram app install ads require installation of SDK (Software Development Kit) into your app system, but does not require submission of website URL.
Nevertheless, link click ads require provision of website URL and if you provide your app URL, your potential customers would be connected to the app store directly. Your website URL will not be exposed to the potential customers, but it is required in order for us to review your ad.
Google UAC & Apple app store:
Google UAC & Apple app store don't require installation of SDK to run app install ads, thus simply need to provide a website URL . Hence, we strongly recommend you to run these if the SDK isn't installed.