How to set a default taxonomy for custom post types?

Please write below code in functions.php :

add_action( 'save_post', 'set_default_category' );
function set_default_category( $post_id ) {
    // Get the terms
    $terms = wp_get_post_terms( $post_id, 'your_custom_taxonomy');
    // Only set default if no terms are set yet
    if (!$terms) {
        // Assign the default category
        $default_term = get_term_by('name', 'taxonomy_slug', 'your_custom_taxonomy');
        $taxonomy = 'your_custom_taxonomy';
        wp_set_post_terms( $post_id, $default_term->term_id, $taxonomy );
    }
}