You must create the app in the Store Connect before uploading it the first time. When you create the app, it is asigned an id. I think thats the iTunes id that you must provide.
If I’m not mistaken, the iTunes URL is formed in with the format:
COUNTRY_CODE: the iso country code (us, es)
APP_NAME: The name of the app, replacing the spaces with dashes. Example: if the app name is “My New App”, in the url will appeard as “my-new-app”
ID: The id present in the app information section of the itunes (it also shows the pack Id, Sku, etc).