Check your in-game notifications in the CS:GO main menu. Boot up CS:GO and load into the main menu. If you got into the CS2 beta test, you'll see a popup telling you so (or else a notification in your left-hand main menu bar with a message about the beta). Just click “Enroll” on the popup, and you're good to go!

Valve will inform selected candidates for the Counter-Strike 2 test in CS:GO's main menu, where you will get a notification. If you see that notification, click “ENROLL” and download CS2. Launch CS:GO after the download is done and choose “Limited Test” to play CS2.