As far as I was aware, it's open to everyone. I only started playing at Stars again in the middle of last year, and only putting in any regular volume from September, but I'm hardly a high-roller as far as rake generation goes.
It's a challenge called the Monthly Poker Challenge that you opt into at the start of each month (there's also an email that comes out confirming it). The actual challenge varies slightly from month to month, but it's basically a redemption target that clears a chest (separate to the chests that fill at the top of the screen, which are additional to this). The earliest ones could be repeated upto 5 times in the month, but the last couple can only be cleared twice (and I won't get close to repeating the January one, as I'm only going to clear it for the first time today).
NB: I've just found some info on it and it's available to everyone in the silver tier or higher. This is the 3rd of 6 tiers in their chest system, so it requires some play to qualify for it, although the tier system itself is a separate rakeback programme. The blue (333 points per chest) and bronze (935 points per chest) levels were pretty easy to clear playing microstakes PLO Zoom, and any sort of regular play will enable you to maintain a level, as it works on a rolling 28 day cycle to clear one chest. NLHE would be a bit more of a grind, but hardly onerous playing Zoom tables.
My bankroll challenge has relied heavily on the various promotions to get off the ground, as I'm essentially targetting break-even play in the early stages (and haven't hit it as yet!)