The APIs are available right now for free. They can be downloaded from the Artifact website.
Pre-orders will open at 9AM (PT) tomorrow. The game itself won't be released until November 28.
The Artifact Preview Tournament will have 128 players competing for a shot at the $10,000 (USD) prize pool.
Hosted by Beyond The Summit and broadcast on SteamTV, the all-draft event will feature a 7-round Swiss bracket on Saturday. Then a top 8 single-elimination bracket will decide the winner on Sunday. Both days begin at 9AM PST.
Valve notes that the tournament will be run through the game's built-in tournament system. This same system will be available to everyone that owns the game.