Http.POST problem

[afterlight]

Hi people, long time no see. I am trying to quickly build a small bot for trading altcoins on Poloniex.com but I am having trouble posting data to the site and retrieving information. The part that I am interested is this:

As stated on their site:

All calls to the trading API are sent via HTTP POST to https://poloniex.com/tradingApi and must contain the following headers:

Key - Your API key.
Sign - The query's POST data signed by your key's "secret" according to the HMAC-SHA512 method.

Additionally, all queries must include a "nonce" POST parameter. The nonce parameter is an integer which must always be greater than the previous nonce used.

and also the command that I want to send is "returnBalances"

So here is my script:

local Key = "TZX6L4AH-7UQ2P6I1-LR0SPQJX-OLZULB64"
local Sign = "1c0a40253e52f9aeab934f0e617ebd80bad16235cfe7271c1a17d8f54ee26d39b001b6ec9e8bf139295052c2c4a1f36b0e34276c5675be341801b0ec187b0589"
local targetURL = "https://poloniex.com/tradingApi"



local strUrlArguments = "?command=%s&nonce=%s"
local strValues = string.format(strUrlArguments,"returnBalances","203");


local PoloniexCal = luacom.CreateObject("WinHttp.WinHttpRequest.5.1");

if (PoloniexCal ~= nil) then

PoloniexCal:Open("POST", targetURL..strValues, 0)
PoloniexCal:SetRequestHeader("Key", Key );
PoloniexCal:SetRequestHeader("Sign", Sign );
PoloniexCal:Send();

if (PoloniexCal.Status == 200) then

local result = PoloniexCal.ResponseText
Dialog.Message("Google's Response", result);

else

Dialog.Message("Error", "Failed To Complete Http Request");

end

PoloniexCal = nil;
collectgarbage();

end

The return I get is

{"error":"Invalid command."}

Don't quite know what to do next. Hope someone can help me. Thanks in advance