Add boost to Smile ElasticSearch category products

Question

I'm trying to add a boost to products in a certain category to be displayed first. Using the Smile extension found in app\code\community\Smile\ElasticSearch\Model\Resource\Engine\Elasticsearch\Query\Abstract.php where the ElasticSearch is built

protected function _assembleQuery()
{
     $query = array('index' => $this->getAdapter()->getCurrentIndex()->getCurrentName(), 'type' => $this->getType());
     $query['body']['query']['filtered']['query']['bool']['must'][] = $this->_prepareFulltextCondition();
     ...
}

I'm trying to add something like:

$query['body']['query']['should'] = [
    'match' => [
        'field' => 'categories',
        'query' => 'id_of_category',
        'boost' => 10,
    ]
 ];

But this is not working. Returning error:

{"error":"SearchPhaseExecutionException[Failed to execute phase [query], all shards failed;... nested: ElasticsearchParseException[Expected field name but got START_OBJECT \"should\"]; 

Can someone help, please?