Requires you to create an alienware account to claim a key and redeemed at https://www.warframe.com/promocode.
There were ~4000 keys remaining when I claimed, the Karak and Skana both come with a potato installed, or just sell them for the free weapon slots if you’ve already mastered them.
I’m not sure if keys are limited (or just released in batches), they were down to ~2600 the last time I checked and now they’re back up to ~4600.
A lot of the older alienware giveaways were plagued by bots claiming keys, not sure if that has ever been fixed.