Alamofire Example.swift

import Alamofire

let urlString = "http://example.org/Webserver.WcfService.Service"
let headers: HTTPHeaders = [
  "Accept" :       "text/xml",
  "Content-Type" : "text/xml; charset=UTF-8",
  "SOAPAction": "http://tempuri.org/Service/GetInfo" //this is important and the value depends on your WSDL, but with our experiments, without this header nothing works
]
let parameters: Parameters = [
  "UserName": "username",
  "Password": "password",
  "firstParameter":  "firstParameter value",
  "secondParameter":  "secondParameter value",
  ]

var req = Alamofire.request(urlString, method: .post, parameters: parameters, encoding: "requestEncoding", headers: headers)

req.responseString{ response in
  debugPrint(response)
  switch response.result{
    case .success( _):
      //process response.data!
    case .failure(let error):
      //process error
  }
}

String Extension.swift

extension String: ParameterEncoding {
  public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
    var request = try urlRequest.asURLRequest()
    let params = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n"
        + "<soap:Envelope\n"
        + "xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\"\n"
        + "xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\"\n"
        + "xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\n"
        + "<soap:Body>\n"
        + "<GetInfo xmlns=\"http://tempuri.org/\">\n"
        + "<UserName xmlns=\"http://tempuri.org/\">\((parameters?["UserName"])!)</UserName>\n"
        + "<Password xmlns=\"http://tempuri.org/\">\((parameters?["Password"])!)</Password>\n"
        + "<FirstParameter xmlns=\"http://tempuri.org/\">\((parameters?["FirstParameter"])!)</FirstParameter>\n"
        + "<SecondParameter xmlns=\"http://tempuri.org/\">\((parameters?["SecondParameter"])!)</SecondParameter>\n"
        + "</GetInfo>\n"
        + "</soap:Body>\n"
        + "</soap:Envelope>\n"
    request.httpBody = params.data(using: .utf8)!
    return request
}