Index Field for Search API

Not sure whether this is a Node Premium Feature but I think so.

In the Select fields to index in the Search API configuration, all sorts of Fields can be chosen to be indexed. Among them Status, Promoted to front page and Sticky in lists - all items that can be chosen in the Publishing options area when creating a new page. The premium content option is also situated in this area but in the Search API Select fields section it is not visible and cannot be indexed.

How can the option to index the Premium Content checkbox from the Publishing options area be added to the Search API Select fields section? Must be easy but I am a bit lost here. Can anyone help out?