how to calculate length of request when using alamofire

In one of our authorization, we have to calculate the length of the content (httpbody) during post method and pass exactly the same content so the service side so service can pickup the same content & format to validate and do the checksum. Most examples that I've searched are shown with Dictionary, how do I calculate post request length beforehand.

btw, we are using alamofire but need to pass more extra data.