I will spare you the prolonged intro, an app that is ideal for any sort of the external giveaway on for example social media is called Gleam.
They have the self-hosted pages-widgets so it doesn't matter if you do not have a website where to place it, a link is enough.
People and companies on Twitter usually hoist giveaways with devices, smartphones, gift cards or redeemable codes for games.
For anyone who wants to attract new users on the Smoke.Io this could be a really good idea. It would be better if we could have an easier signup.
The website and an app is already well-known, so there is no problem in finding the proper audience.