How to send a HTTP POST Request in Delphi XE6 using WinInet api

You have some errors in your code. Most notably, you are mixing string types, and you are transmitting your Data parameter as-is in its original UTF-16 format, which the server is not expecting since you are not sending a charset=utf-16 value in the request's Content-Type header. You need to ensure the Data is in the correct format before you can send it.

Try something more like this instead:

procedure TForm2.WebPostFormData(const UserAgent: String; const Server: string; const Resource: String; const Data: TStrings);
var
  FormData: TStringList;
  PostData: UTF8String;
  I: Integer;
  BufStream: TMemoryStream;
  Resu: AnsiString;
begin
  FormData := TStringList.Create;
  try
    FormData.NameValueSeparator := '=';
    FormData.LineBreak := '&';
    for I := 0 to Data.Count-1 do
    begin
      // TODO: URL-encode the name and value...
      FormData.Add(Data.Names[I] + '=' + Data.ValueFromIndex[I]);
    end;
    PostData := UTF8String(FormData.Text);
  finally
    FormData.Free;
  end;

  BufStream := TMemoryStream.Create;
  try
    WebPostData(UserAgent, Server, Resource, PAnsiChar(PostData), Length(PostData), 'application/x-www-form-urlencoded; charset="utf-8"', BufStream);
    SetString(Resu, PAnsiChar(BufStream.Memory), BufStream.Size);
  finally
    BufStream.Free;
  end;

  ShowMessage(String(Resu));
end;

procedure TForm2.WebPostData(const UserAgent: String; const Server: string; const Resource: String; const Data: Pointer; DataSize: UInt32; const ContentType: String; Response: TStream);
var
  hInet: HINTERNET;
  hHTTP: HINTERNET;
  hReq: HINTERNET;    
  Heade: String;
  Buffer: array[0..1023] of Byte;
  BytesRead: DWORD;
const
  accept: packed array[0..1] of PChar = (PChar('*/*'), nil);
begin
  hInet := InternetOpen(PChar(UserAgent), INTERNET_OPEN_TYPE_PRECONFIG, nil, nil, 0);
  if hInet = 0 then RaiseLastOSError;
  try
    hHTTP := InternetConnect(hInet, PChar(Server), INTERNET_DEFAULT_HTTP_PORT, nil, nil, INTERNET_SERVICE_HTTP, 0, 1);
    if hHTTP = 0 then RaiseLastOSError;
    try
      hReq := HttpOpenRequest(hHTTP, PChar('POST'), PChar(Resource), nil, nil, @accept, INTERNET_FLAG_KEEP_CONNECTION, 1);
      if hReq = 0 then RaiseLastOSError;
      try
        Heade := 'User-Agent: ' + UserAgent + #13#10 +
                 'Accept-Language: en-us,en;q=0.5'#13#10 +
                 'Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7'#13#10 +
                 'Content-Type: ' + ContentType + #13#10 +
                 'Keep-Alive: 300'#13#10;

        if not HttpAddRequestHeaders(hReq, PChar(Heade), Length(Heade), HTTP_ADDREQ_FLAG_ADD) then RaiseLastOSError;

        if not HTTPSendRequest(hReq, nil, 0, Data, DataSize) then RaiseLastOSError;

        repeat
          if not InternetReadFile(hReq, @Buffer, SizeOf(Buffer), BytesRead) then RaiseLastOSError;
          if (BytesRead = 0) then Break;
          if Response <> nil then
            Response.WriteBuffer(Buffer, BytesRead);
        until False;
      finally
        InternetCloseHandle(hReq);
      end;
    finally
      InternetCloseHandle(hHTTP);
    end;
  finally
    InternetCloseHandle(hInet);
  end;
end;

Alternatively, consider switching to Indy's TIdHTTP component and let it do the work for you:

uses
  ..., IdGlobal, IdHTTP;

procedure TForm2.WebPostFormData(const UserAgent: String; const Server: string; const Resource: String; const Data: TStrings);
var
  HTTP: TIdHTTP;
  Resu: String;
begin
  HTTP := TIdHTTP.Create(nil);
  try
    HTTP.Request.Accept := '*/*';
    HTTP.Request.UserAgent := UserAgent;
    HTTP.Request.AcceptLanguage := 'en-us,en;q=0.5';
    HTTP.Request.AcceptCharset := 'ISO-8859-1,utf-8;q=0.7,*;q=0.7';
    HTTP.Request.ContentType := 'application/x-www-form-urlencoded';
    HTTP.Request.Charset := 'utf-8';
    HTTP.Request.Connection := 'keep-alive';
    HTTP.Request.CustomHeaders.Values['Keep-Alive'] := '300';

    Resu := HTTP.Post('http://' + Server + Resource, Data);
  finally
    HTTP.Free;
  end;

  ShowMessage(Resu);
end;

procedure TForm2.WebPostData(const UserAgent: String; const Server: string; const Resource: String; const Data: Pointer; DataSize: UInt32; const ContentType: String; Response: TStream);
var
  HTTP: TIdHTTP;
  DataStrm: TIdMemoryBufferStream;
begin
  HTTP := TIdHTTP.Create(nil);
  try
    HTTP.Request.Accept := '*/*';
    HTTP.Request.UserAgent := UserAgent;
    HTTP.Request.AcceptLanguage := 'en-us,en;q=0.5';
    HTTP.Request.AcceptCharset := 'ISO-8859-1,utf-8;q=0.7,*;q=0.7';
    HTTP.Request.ContentType := ContentType;
    HTTP.Request.Connection := 'keep-alive';
    HTTP.Request.CustomHeaders.Values['Keep-Alive'] := '300';

    DataStrm := TIdMemoryBufferStream.Create(Data, DataSize);
    try
      HTTP.Post('http://' + Server + Resource, DataStrm, Response);
    finally
      DataStrm.Free;
    end;
  finally
    HTTP.Free;
  end;
end;