Here is the documentation for publishing rest services with Mendix native functionality.
Also there is another way to do it with the rest module. Here is a blog post about.
Edit: I haven't had a chance to try out the new publish rest service function. I know how to do it with the rest module. But maybe in your microflow you have to return this object. Its in the system module. Try creating it and returning it at the end of your microflow.