When you purchase your first supporter pack that includes an Early Access key, the key will be automatically redeemed on the account used for the purchase. There's nothing else you need to do—just download the game client when it's available and log in on launch day!
If you have purchased a key or have been granted access for one PC game client (Stand-alone, Steam, or Epic) but wish to play on another game client, you can claim a key to unlock access to that client. These keys are available at my-account/early-access and can be redeemed on the relevant platform's store. Please be aware that these keys alone do not grant early access to your account; they merely open the client in your library.
For example, if you purchase a Supporter Pack through the Path of Exile website, you will be able to claim a key to unlock the client in your Steam Library.