gistfile1.php

/*
* For latest WooCommerce Rest API
 * Add a referanse field to the Order API response.
*/ 
function prefix_wc_rest_prepare_order_object( $response, $object, $request ) {
	// Get the value
	$referanse_meta_field = ( $value = get_post_meta($object->get_id(), '_billing_referanse', true) ) ? $value : '';

	$response->data['referanse'] = $referanse_meta_field;

	return $response;
}
add_filter( 'woocommerce_rest_prepare_shop_order_object', 'prefix_wc_rest_prepare_order_object', 10, 3 );

/*
 * For Legacy API
 * Add a referanse field to the Order API response.
*/ 
function prefix_wc_api_order_response( $order_data ) {
	// Get the value
	$referanse_meta_field = ( $value = get_post_meta($order_data['id'], '_billing_referanse', true) ) ? $value : '';

	$order_data['referanse'] = $referanse_meta_field;

	return $order_data;
}
add_filter( 'woocommerce_api_order_response', 'prefix_wc_api_order_response', 10, 1 );


/*
For customer response
*/
add_filter( 'woocommerce_rest_prepare_customer', 'th35tr_override_rest_response_customer', 10, 3 );
function th35tr_override_rest_response_customer($response,  $user_data,  $request){
	$meta_value_1 = ( $value = get_user_meta( $user_data->ID, 'field_name_1' , true ) ) ? $value : '';
	$response->data['field_name_1'] = $meta_value_1;
	return $response;
}

/*
For subscription response
*/
function th25er_modify_wc_rest_subscription_object( $response, $object, $request ) {
	// Get order ID
	$order_id = $response->data['parent_id'];
	// Get the value
	$value = ( $value = get_post_meta($order_id, 'discord_username', true) ) ? $value : '';

	$response->data['billing']['discord_username'] = $value;

	return $response;
}
add_filter( 'woocommerce_rest_prepare_shop_subscription', 'th25er_modify_wc_rest_subscription_object', 10, 3 );