The latest release of macOS High Sierra 10.13.5 brings the latest update for support for Messages in iCloud.
Apple has been testing and working on bringing the feature Messages in iCloud to macOS High Sierra since June 2017, Apple also recently released iOS 11.4, which also features support for Messages in iCloud, AirPlay 2, and HomePod stereo pairing.
Messages in iCloud will now allow users to store their messages in iCloud rather than on their devices, macOS 10.13.5 update is 2.12GB and requires the user to restart the Mac in order to install the update, (it is suggested to first perform a backup data just in case.)
To install the update, first launch the App Store, then click Updates, which should allow for the installment, in case the recent 10.3.5 does not appear, try reloading the Updates page by pressing Command-R on the keyboard. After installing, in order to activate Messages in iCloud, first launch the Messages app, click on Messages>Preferences, Click Accounts, and select Enable Messages in iCloud.