First, you need a paid project, or at least one that has a file with a price.

In your Dashboar -> promotion tab -> Create a new sale or bundle -> Earnings goal

ps: I don't know if it can be done with games that only have free content, since when creating the sale, you can only mark projects that have a price. But you should try or add a file with a price to the project that serves as a thank you to the one who helps you reach the goal.