I'm attaching an example of creating a News Item, using Sitefinity's services. Mind that you have to specify credentials of a user that has create/edit permissions for news items, so that the service can be consumed. You can also check this blog post for more general information on Sitefinity's RESTful services:
Considering the required fields - they are the same as they would be if you created a NewsItem from the backend. It should have a Title, UrlName, Content, PublicationDate and an Author. Considering the error that is thrown to you, I think the reason is this line:
Please check the example which shows how to log in Sitefinity and consume the service for creating a NewsItem. You can find all the documentation link and how to make use of the service's help page in the blogpost link above.
Let me know if you have further problems with this.
the Telerik team
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