I believe all you need to do is enter your idea on the Lisk forums, in their decentralized app section. Although looking at how this is set up, one can see potential problems.

Nothing to stop someone else from simply stealing an idea you post, sprucing it up a bit better than you, and by the contest rules, end up winning the contest. Or the opposite... someone having a great idea early on, posting it before you, even if you could code the entire thing by yourself in a weekend.

It's sort of a weird way to run a contest, especially since I expect a lot of repeated ideas ... and only three winners.