A reward can not be created or exist on its own. A reward is a type of item that is triggered when a reward is attached to a Contest or an Achievement. Multiple rewards can be created at the same time.
A Reward within a Contest
A Reward within an Achievement
To create a Reward you can do it by creating a Contest and within the contest Post body parameters you need to add the Reward parameters.
You can find the Post body parameters and how to create a reward or multiple rewards within a Contest in the link below
POST /api/:space/competitions/:competitionId/contests
Or if you have a pre created contest without a reward and you want to add a Reward or multiple Rewards to it you can do it by updating the Contest in the link below
PUT /api/:space/competitions/:competitionId/contests/:contestId
To create a Reward you can do it by creating an Achievement and within the achievement Post body parameters you need to add the Reward parameters.
You can find the Post body parameters and how to create a reward or multiple rewards within an Achievement in the link below
Or if you have a pre created achievement without a reward and you want to add a Reward or multiple Rewards to it you can do it by updating the achievement in the link below