To get one, you’ll need a special code from us, FBTB, or Brickset. Here are the details from LEGO:
We have discovered a limited quantity of LEGO Star Wars™ Shadow ARF Trooper minifigures in our warehouse! Due to the very limited quantity we would like to give the fan community the first chance to receive these as bonus gift on any LEGO Star Wars holiday purchases that meet the Free Shipping threshold of $99.
The offer will be available Nov 7th – 21st or while supplies last. To add the LEGO Star Wars™ Shadow ARF Trooper to your eligible order simply enter code 4TBB (US) / TBBO (CA) at checkout.
Unfortunately, warehouses elsewhere in the world didn’t have a lost stash of ARF Troopers, so this is limited to North America.