In iTunes Connect, navigate to My Apps and select your application. You will now be in the App Information page which will list the iOS App Store ID under General Information, see screenshot below. In this example, the iOS App Store ID is 1237865618.
If you have not yet created the application in your iTunes Connect account, you can add a new app by clicking on the + symbol, see screenshot below. After creating a new app, you will then be able to access the iOS App Store ID by following the above steps.