Custom HTTP Headers with Swift 3 or 4 and Alamofire 4.0-4.7: See https://grokswift.com/custom-headers-alamofire4-swift3/ for explanations

Custom HTTP Headers with Swift and Alamofire.swift

// MARK: - Adding a header to a single request
func doRequestWithHeaders1() {
  let headers: HTTPHeaders = [
    "X-Mashape-Key": MY_API_KEY,
    "Accept": "application/json"
  ]

  Alamofire.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh", headers: headers)
    .responseJSON { response in
    debugPrint(response)
  }

  // MARK: - To make sure the headers are sent, use debugPrint on the request
  let request = Alamofire.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh", headers: headers)
    .responseJSON { response in
    debugPrint(response)
  }
  debugPrint(request)
}

// MARK: - Adding headers to all requests in the session

// store sessionManager so it doesn't get deallocated while we're waiting for the network call to finish
var sessionManager: Alamofire.SessionManager?
func doRequestWithHeaders2() {
  // get the default headers
  var headers = Alamofire.SessionManager.defaultHTTPHeaders
  // add your custom headers
  headers["API-Version"] = "2.0"
  headers["X-Mashape-Key"] = MY_API_KEY

  // create a custom session configuration
  let configuration = URLSessionConfiguration.default
  // add the headers
  configuration.httpAdditionalHeaders = headers

  // create a session manager with the configuration
  self.sessionManager = Alamofire.SessionManager(configuration: configuration)

  // make calls with the session manager
  sessionManager?.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh")
    .responseJSON { response in
    debugPrint(response)
  }
}

// MARK: - Add headers when using a URLRequest
func doRequestWithHeaders3() {
  if let url = URL(string: "https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh") {
    var urlRequest = URLRequest(url: url)
    urlRequest.httpMethod = HTTPMethod.get.rawValue

    urlRequest.addValue(MY_API_KEY, forHTTPHeaderField: "X-Mashape-Key")
    urlRequest.addValue("application/json", forHTTPHeaderField: "Accept")

    Alamofire.request(urlRequest)
      .responseJSON { response in
        debugPrint(response)
    }
  }
}

// Alternatively:
func doRequestWithHeaders4() {
  if let url = URL(string: "https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh") {
    var urlRequest = URLRequest(url: url)
    urlRequest.httpMethod = HTTPMethod.get.rawValue

    var headers: HTTPHeaders
    if let existingHeaders = urlRequest.allHTTPHeaderFields {
      headers = existingHeaders
    } else {
      headers = HTTPHeaders()
    }
    headers["X-Mashape-Key"] = MY_API_KEY
    headers["Accept"] = "application/json"
    urlRequest.allHTTPHeaderFields = headers

    let request = Alamofire.request(urlRequest)
      .responseJSON { response in
        debugPrint(response)
    }
    debugPrint(request)
  }
}

// MARK: - Use an Alamofire RequestAdapter to add headers to all calls in the session

class MashapeHeadersAdapter: RequestAdapter {
  func adapt(_ urlRequest: URLRequest) throws -> URLRequest {
    var urlRequest = urlRequest
    
    urlRequest.setValue(MY_API_KEY, forHTTPHeaderField: "X-Mashape-Key")
    urlRequest.setValue("application/json", forHTTPHeaderField: "Accept")
    
    return urlRequest
  }
}

func doRequestWithHeaders5() {
  // Then to make the call:
  // get a session manager and add the request adapter
  let sessionManager = Alamofire.SessionManager.default
  sessionManager.adapter = MashapeHeadersAdapter()

  // make calls with the session manager
  let request = sessionManager.request("https://mashape-community-urban-dictionary.p.mashape.com/define?term=smh")
    .responseJSON { response in
      debugPrint(response)
    }
  debugPrint(request)
}