Simply tap ‘Cross-Sync’ from the Start Screen. If you haven’t logged in already, a web popup will open to take you through the log in.
This will connect your App Store/Google Play account to your Steam account. Doing so will also enable Cross-Purchase. For more information on Cross-Purchase, see here: https://community.sigames.com/faq/football-manager-touch-2018/store-and-unlockables/what-is-the-cross-purchase-system-r571/
Edited by Neil Brock