I will provide so website you can easily get key to free use on Steam.
Get Horizon Zero Dawn Steam / CD Key for Free
Here is a list of few website that are providing steam key for horizon zero dawn at free of cost. Just go there and signup to the website and enjoy free steam key.
This is a website that you can use to achieve your free cd key for the game. You have to just gain some coins.
This is another website providing steam keys. What are you waiting for? get your key now.