The listing, Quake Champions Beta has ended.
After you get a key, go to Bethesda and make an account (also verify your email):
https://account.bethesda.net
Activate the key here: https://account.bethesda.net/en/settings/redeem-code
Download link to a client will appear after you activate the code, download the client, login and start downloading quake, when the beta starts (April 20) you can play it.