1. Fiddler | Link: https://www.telerik.com/download/fiddler
1. Xbox App | Link: https://www.xbox.com/en-US/apps/xbox-app-on-pc (Just download of the app store)
First open Fiddler and go to: 'Tools > Options > HTTPS' and check 'Decrypt HTTPS Traffic'.
Next click WinConfig and scroll to the bottom and check every option which is releated to Xbox.
And that is it, keep fiddler open before following the next step.
Next load up Xbox app with fiddler open and running. Now once Xbox app is running simply click on the game on Xbox app and go back to fiddler and look for a request which starts with 'catalog'.
Then go to the response section and go to the 'JSON' section and look for 'id=' and/or 'idType=', that is the games Title ID.
(It should look like the following)
(List of over 150 Title ID's can be found here)