The current way WP Dispensary adds in the custom flowers_category taxonomy to the Flowers REST API endpoint

wpd-rest-api-flowers-category.php

<?php
/**
 * Add Category taxonomy for the Flowers Custom Post Type
 */
function flowers_category( $data, $post, $request ) {
	$_data                     = $data->data;
	$_data['flowers_category'] = get_the_term_list( $post->ID, 'flowers_category', '', ' ', '' );
	$data->data                = $_data;
	return $data;
}
add_filter( 'rest_prepare_flowers', 'flowers_category', 10, 3 );