FIFA 18 is available in The Vault, for EA Access subscribers. If you are not seeing it in the app, try downloading it from here or head over to the Xbox Store on your console and search for it.
For those of you that might still be unaware, EA Access is an Xbox One exclusive, allowing members to try upcoming EA games prior to release, offering 10% off all EA digital purchases and more. It also gives users access to The Vault, which houses a growing library of EA games subscribers can play as much as they want. Users can subscribe to EA Access for $4.99 per month, or $29.99 for a full year.