let URL = Foundation.URL(string: "http:....")!
let mutableURLRequest = NSMutableURLRequest(url: URL)
mutableURLRequest.httpMethod = "POST"

let parameters = ["UserName": "Teste"]
  do{
    mutableURLRequest.httpBody = try JSONSerialization.data(withJSONObject: parameters, options: JSONSerialization.WritingOptions())
    }catch{}

mutableURLRequest.setValue("application/json", forHTTPHeaderField: "Content-Type")

Alamofire.request("https").responseJSON{response in....}