When a user highlights a block of text, they are given the option in the contextual menu to “Search with {Search Engine}” where {Search Engine} refers to the title of the user-selected default search engine in Safari preferences. Upon selecting this menu option, the query is passed on to the search engine to be executed. However, the domain of the search engine is not recorded in resource load statistics as having had user interaction as a result of the performed query.
<rdar://problem/33971309>
Interactions with application (e.g., Safari) UI (such as Context Menus) are outside the scope of the WebKit project. Your best option is to communicate through Apple Developer Relations referencing the Radar generated by this bug report (<rdar://problem/33971309>). That said, we do not assign user interaction credit for activity taking place outside the scope of the browser page by design. Consequently, the feature currently works as designed.