Post 2 Post Site Search

I’m trying to filter search terms bases on categorisation created by posts 2 posts.

I’m using the generic site search and then I’ve created a search filter

function my_filter_the_search($query){

    $site_category = filter_input(INPUT_GET, 'site_category', FILTER_SANITIZE_STRING);

        $args = array(
            'connected_type'        => 'sitecat_to_product',
            'connected_items'       => $query
        );

        $query->set( 'query', $args );


        return $query;

}

to run before the results are displayed.

Does anyone know how I extend the search to then filter for all posts that contain the search term but are then attached to the specific post 2 posts