@WebMethod(operationName = "autentica")
public String autentica() {
    MessageContext mContext = wsContext.getMessageContext();

    Map http_headers = (Map) mContext.get(MessageContext.HTTP_REQUEST_HEADERS);
    List userList = (List) http_headers.get("Username");
    List passList = (List) http_headers.get("Password");

    String username = "";
    String password = "";

    if(userList != null) {
        username = userList.get(0).toString();
        System.out.println("User: " + userList.get(0).toString());
    }

    if(passList != null) {
        password = passList.get(0).toString();
        System.out.println("Pass: " + passList.get(0).toString());
    }

    if (username.equals("admin") && password.equals("admin")){
        return "Hello World JAX-WS - Valid User!";
    }else{
        return "Unknown User!";
    }
}

$soap.setCredentials("admin","admin");
$soap.post(url, "autentica").then(
    function(response) {
        console.log(response);
    }
);

<S:Envelope xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
   <S:Body>
      <ns2:autenticaResponse xmlns:ns2="http://service.natal.rn.gov.br/">
          <return>Unknown User!</return>
      </ns2:autenticaResponse>
   </S:Body>
</S:Envelope>

$soap.setCredentials("username","password");

xmlHttp.setRequestHeader("Authorization", "Basic " + SOAPClient._toBase64(SOAPClient.userName + ":" + SOAPClient.password));