Add custom taxonomy to custom search for posts

you should go through all wpQuery docs for easily understand this.
To do this task use this for for making query:

$args = array(
    'post_type' => 'your_custom_postType',
    'tax_query' => array(
        array(
            'taxonomy' => 'your texonomy ', //like category, actor, people etc.
            'field'    => 'slug',
            'terms'    => 'your_term', //like rock,bread, tom
        ),
    ),
);
$query = new WP_Query( $args );