During the month of December 2017, anyone can get a free helm MTX by watching any streamer play Path of Exile on their Twitch stream.This promotion allow anyone to get a free helm, but each Path of Exile account can only get 1x free hat MTX during this time. When you win it, you get a random draw from any of the 23 hats / helmets that are part of this promotion.
What you need to do is to log in to both your Twitch account as well as your Path of Exile account on the same browser. Now head over to this webspage to link your two accounts together.
Now that you have connected Twitch and PoE, you can head over to any PoE stream to have a chance at getting the free hat / helmet. Every 5 minute after you begin to watch, a draw takes place. At this draw you have a 1 % chance to get your free helmet! This means that you can get lucky and get one after 1 minutes, or unlucky enough to have to wait 15+ hours.
The average should be between 8 and 9 hours, so be prepared to watch Ziz or any other PoE streamer for a long time. However, you can always idle it by keeping the stream running after you go to bed or something like that. You are in the draw pool as long as you are watching a streamer that is online and that is streaming PoE. The streamers can not decide to not participate, so feel free to watch whichever PoE streamer you prefer.
You get a message on your PoE account when you win, and that tells you what type of hat you won. The message looks like this:
To read more about this promotion, head over to this post on the PoE forums.