Extra infos: At https://partner.steamgames.com/doc/webapi/ISteamUserStats
there's a POST method called SetUserStatsForGame (at the bottom) with the warning:
Quote: "NOTE: This call requires the publisher API key that owns the specified App ID to use this method. As such this API MUST be called from a secure server, and can never be used directly by clients!"
Achievements are part of user stats, rather than being their own entity. It would appear to be important to get the user stats bundle first, add the achievement, and then save - otherwise existing stats (inc. other achievements) would be deleted.
The heavy-lifting needs to be done on one's server in the server-side language of one's choice. Obviously outside the remit of AGK. The necessary functions (HTTP, SHA, unixtime, etc.) for the game to talk to the server (yours not Steam) are already present.
If/when I get round to implementing this myself, I'll post an update (distant future at the moment).