ASP .NET - POST request method

[Trok]

Hi everybody,
I need help with creating correct POST request to website working with ASP .NET.
May begin that my request was contructed by using sniffer results.
Here it is:

request.setRawHeader("Origin:","http://www.md5decrypter.co.uk");
    request.setRawHeader("X-Requested-With:","XMLHttpRequest");
    request.setRawHeader("Cache-Control:","no-cache");
    request.setRawHeader("X-MicrosoftAjax:","Delta=true");
    request.setRawHeader("Accept:","*/*");
    request.setRawHeader("Referer:","http://www.md5decrypter.co.uk/sha1-decrypt.aspx");
    request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded; charset=UTF-8");
    request.setRawHeader("Accept-Language:","pl-PL,pl;q=0.8,en-US;q=0.6,en;q=0.4");
    request.setRawHeader("Accept-Charset:","ISO-8859-2,utf-8;q=0.7,*;q=0.3");
 
    postData.addQueryItem("ctl00$ScriptManager1", "ctl00$MainContent$btnDecrypt");
    postData.addQueryItem("ctl00$MainContent$txtHashes", "f41b59b297ded4f90ba61ee3c5ff18220ae55c66");
    postData.addQueryItem("ctl00$MainContent$txtCaptcha", sCaptcha);
    //postData.addQueryItem("__EVENTTARGET", "ctl00$MainContent$btnDecrypt");
    //postData.addQueryItem("__VIEWSTATE", sViewState);
    postData.addQueryItem("__EVENTVALIDATION", sEventValidation);
    postData.addQueryItem("__ASYNCPOST", "true");

request.setRawHeader("Origin:","http://www.md5decrypter.co.uk");
    request.setRawHeader("X-Requested-With:","XMLHttpRequest");
    request.setRawHeader("Cache-Control:","no-cache");
    request.setRawHeader("X-MicrosoftAjax:","Delta=true");
    request.setRawHeader("Accept:","*/*");
    request.setRawHeader("Referer:","http://www.md5decrypter.co.uk/sha1-decrypt.aspx");
    request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded; charset=UTF-8");
    request.setRawHeader("Accept-Language:","pl-PL,pl;q=0.8,en-US;q=0.6,en;q=0.4");
    request.setRawHeader("Accept-Charset:","ISO-8859-2,utf-8;q=0.7,*;q=0.3");

    postData.addQueryItem("ctl00$ScriptManager1", "ctl00$MainContent$btnDecrypt");
    postData.addQueryItem("ctl00$MainContent$txtHashes", "f41b59b297ded4f90ba61ee3c5ff18220ae55c66");
    postData.addQueryItem("ctl00$MainContent$txtCaptcha", sCaptcha);
    //postData.addQueryItem("__EVENTTARGET", "ctl00$MainContent$btnDecrypt");
    //postData.addQueryItem("__VIEWSTATE", sViewState);
    postData.addQueryItem("__EVENTVALIDATION", sEventValidation);
    postData.addQueryItem("__ASYNCPOST", "true");

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Unfortunately, I don't receive satisfying outcome. Also, can't understand why in case of sending __EVENTTARGET or __VIEWSTATE, getting this kind of error:

1|#||4|52|pageRedirect||%2ferror.aspx%3faspxerrorpath%3d%2fsha1-decrypt.aspx|

1|#||4|52|pageRedirect||%2ferror.aspx%3faspxerrorpath%3d%2fsha1-decrypt.aspx|

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I can add that my app in the first place, download __VIEWSTATE and __EVENTVALIDATION after connection to website.
Where can be a problem, and why it happen so as above.

Thanks in advance,
Trok