Set plugin-values when creating post via REST-API

I’ve found the problem, why it’s not working with the categories.
I don’t know why, but when creating a post over the API, the post is published before the category is correctly set.

Now, I’m creating drafts (already with correct category). With a second PUT-Request I set the status to “publish” and publish my posts. Now the category is correctly set and only the expected posts are published to Facebook.