If using =Dump(Connector("Sistrix.DomainOverview";B3;"domain";"de";"KwCountSeo";TRUE)) the search runs in the basic keyword set. To search in the extended keyword set, there should be an additional API parameter extended=true, like i do it with XpathOnUrl:
=XPathOnUrl(VERKETTEN("https://api.sistrix.com/domain.kwcount.seo?api_key=myKey&extended=true&domain="&C3);"response/answer/kwcount.seo";"value").
For me its enough XpathOnUrl - maybe someone will need extended keyword set...