Question: fulltext search on matrix field, different from entry cp search?

I'm trying to create a filter for the search query: productDescription:sometext. This works fine in the default searchbar, but when using Searchit with this twig:

{{ ({
        filter: {
            {# same result with '*sometext*' #}
            productDescription: 'sometext'
        },
        label: "My Label"
    })|json_encode()
}}

productDescription is a matrix field with a redactor field with the handle text in which it should search.

Should I use a different syntax in this case?
Thanks.