We can generate the app link before the application being submitted to iTunes app store for review.
Here you need to follow the below mentioned steps.
(1) First login to your app store developer account and go to iTunes Connect.
(2) Then go to the option "Manage Your Applications".
(3) Now create new application here and add meta data and all the information about the application like Description, Category, Keywords, Price, etc. which can be edited later. Here you just remaining with the binary submission which you can do after adding the app store link.
(4) After creating your application here from the "Manage Your Application" section, you will get the Application listed. Now you select the app and then in the top menu click on More -> View on App Store.
(5) Now copy the link opened in the browser and that is the link of your application.
https://itunes.apple.com/app/appname/id123456789?ls=1&mt=8- Where appname = name of the application- id123456789 = it will be the ID of your App

These are the simplest steps to create APP store link before the app being submitted to app store actually.
Let me know if you need more help.