The title says it all-
where can I post a contest for a UI for my game?
For a contest, some sort of advertising one, you need so many viewers and contesters as possible.
When you want only few of them, you could post it anywhere.
For success you have to generate some listeners before, hardest part.
Social media is good for that, when you have many followers, reposting your stuff in different channels.
The prize has to be interesting. All over one Billion prize cash, creates massive listener audience on the fly.