So for people who alreadu have Horizons apprently it's
To redeem a Steam key for Elite Dangerous (and Horizons, once it's released on Steam) please follow these steps:
Log into the Elite Dangerous store and go to your account dashboard.
Click on ‘Partner Keys’ and then click ‘Claim an Elite Dangerous’ key.
Log in to Steam and follow the prompts.
Click "Claim a key for Elite Dangerous".
You can now add this key to your Steam account as you would for any other game. Check out this guide for details.
Once you've done this, you should be automatically able to install Elite Dangerous or Elite Dangerous: Horizons via Steam and use the Steam Wallet to make in-game purchases here.
Please note: The key you generate will add your games to any Steam library but WILL NOT GRANT ACCESS TO ELITE: DANGEROUS without a registered Frontier account and a copy of the game. For this reason, please do not give your Steam key away to a friend! They won’t be able to play without having already bought the game from Steam or from Frontierstore.net.
OK, so I have Premium Beta, and I go to "partner Keys", but at the moment all there is is Elite Dangerous, no Horizons.
Oh well, I'm sure it will be sorted soon. But not the smoothest launch unfortunately.