How do I get the Url in a POST RestLet? In a GET RestLet it works, but in a POST I only get the body of the response.

you dont

Thanks

FWIW, I think you can still get the url in a POST using a Suitelet instead of RESTlet.

Yeah, looks like your right. But I guess RESTlet is the better choice when making a REST api

It depends on your needs. If you need to have more control over the url/params/body processing then a RESTlet isn't a good choice. RESTlets don't expose a REST api anyway.