Assigns a news item to a newsfeed.
Publish date of the news item on the newsfeed, use this field if you want to set a publish date in the past (e.g. when importing existing news items). On write, this field will be ignored if the news item state is "draft", and a date in the future is rejected with a 400. Null while the news item isn't live.
Example:1674917488
{ "newsfeed_id": 198313, "published_at": 1674917488 }