Add a new content item to the Set at the given position. If no position is given, the new item is stored at position 0, and treated as "unplaced". Unplaced content is not returned by the standard sets API even when scheduled: only the Sets API for CMS will return it.
A content_url is mandatory. Additional metadata fields, relating to the the contents containment within the set, will be available depending on the type of that content.
If the given set does not exist a 404 Not Found response is returned.
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