if( ! function_exists( 'videotube_pre_query_search_result' ) ){
    /**
     * Filter the query of search result
     * @param $query
     */
    function videotube_pre_query_search_result( $query ) {
        if( $query->is_search() && $query->is_main_query() ){
            $query->set( 'tax_query',
                array(
                    array(
                        'taxonomy' => 'categories',
                        'field' => 'id',
                        'terms' => array( 243 ), // an array of video category, e.g: array( 1,2,3,4 )
                        'operator' => 'IN'
                    )
                )
            );
        }
    }
    add_action( 'pre_get_posts', 'videotube_pre_query_search_result', 10, 1 );
}