Result Source - return results for search query in specific sites

Hi!

I would like to build a result source that returns results from around 60 sites only in our tenant. 

I was trying to think of a way to identify the sites in question, and found I could use the site description, so I've added ows_SiteDescription to managed property RefinableString111. 

How (and is it possible) to combine this in the search query builder for a custom result source, so the query would be to find content based on {searchTerms} but only in sites where the description of a site contains a fixed string.  

Thank you!