the documentation says
object containing fields whose names are the indexes to where objects are mentioned in the message field; each field in turn is an array containing an object with id, name, offset, and length fields, where length is the length, within the message field, of the object mentioned
what’s missing though is how do we need to encode it? should it be a JSON object?
As far as I’m aware, that’s a read-only field for reading the list of pages/profiles tagged in a post, did you find documentation suggesting it was writeable?