Expose search fields to Search Manager

I think SearchManger is searching only in Title, Description,Content. How can I add more search fields to to SearchManager. I know if I use search results widget, I could go into advanced section and could add search fields. But I need to use Search Manager. How can I do this with SearchManager. Any help would be greatly appreciated.

I would also like some information on how to use the search manager.  sitefinity documentation is very lacking, as usual.