This post is a very old one. It is no longer how we distribute free copies of Receiver.
Overgrowth only comes with Receiver when you buy it by itself on the Humble store or on the Steam store.
Overgrowth does not come with Receiver when you get it in a bundle (such as the Humble Freedom Bundle a year ago, or the Humble Monthly Bundle that we're in now).
If you got the game through a trade/from a friend, or bought it on an unauthorized store like G2A, we have no idea what kind of key you got. It's up to the luck of the draw in that case. You might get Receiver, you might not.
If you got Overgrowth in a bundle, or some unknown source (trade, friend, unauthorized third party key reseller), then you can buy Receiver here:
https://www.humblebundle.com/store/receiver
If you bought Overgrowth by itself, on the Humble store or the Steam store, and did *not* get it in a bundle, then it should already be in your purchases. When you register your key on Steam, it should automatically show up in your game library, and you can install it from there. If you bought it on the Humble store, it should show up in your game library.