If you want to join Humankind's closed beta, the main method is to simply prepurchase the game via the official storefronts, Steam and Epic Games, and Stadia.
Upon purchase, you will receive access to the closed beta via a key. That said, once you are in the beta, you will be able to play from June 13th to June 21st - after June 21st, you will not be able to play.
However, if you don't wish to go with the pre-order method, you can also earn a beta key through twitch drops. To use this method, you will need to create a Games2Gather account and then link it to your twitch.
Related: Epic Games Store Exclusives List
After this, all you need to do is watch streamers playing Humankind with drops activated for around four hours. Doing so should get you a key after that amount of time, and once you get your key, all you need to do is activated it, and you will be able to play.
However, how long can you even play? Well, once you are in the beta, you will be able to play through five ages until June 21st and experience most of what the game has to offer, such as building cities and creating units.
Stay tuned for more coverage of Humankind at Pro Game Guides