You'll need to be a Halo Insider to be eligible to play.
Update: Halo Anniversary PC testing will start later this month, with Rings 1 and 2 testing ahead of Ring 3 testing that will begin in February.
“Our goal is to kick off flighting in January for Rings 1 and 2 but it’s possible it could end up slightly delayed,” 343 advised in a new update. “Recently, a new crash has been identified in Halo Wars 2 on PC that is stopping play for keyboard and mouse players.”
Original story [22 December 2019]: Halo Anniversary PC testing is on track to begin in January 2020, 343 Industries has recently confirmed.
“We are currently in a pre-alpha phase and have been making good progress against many of the same features you see in Halo: Reach today,” a new post reads.
Those features — to be added to Halo: The Master Chief Collection on PC — include mouse and keyboard support, FOV support, aspect ratio support, a HUD up-res, VFR supportm input latency reduction and PC optimisations and customisations.
“If work continues on its current trajectory, we are looking good to start flights in January,” 343 concluded. Those who hope to take part will need to be signed up for the Halo Insider program.