Adding custom post meta to Woocommerce REST API V1

functions.php

function ar_add_post_meta_to_order_api_response( $response ) {

	$data = $response->get_data();

	// Adding your custom field:
	$data['postmeta'] = get_post_meta( $data['id'] );

	$response->set_data( $data );

	return $response;
}

add_filter( 'woocommerce_rest_prepare_shop_order', 'ar_add_post_meta_to_order_api_response' );