how can get cookie header data from web page in unity?

I have register and login page in unity3d and my website page , when I POST my register or login data from unity3d to my website(magento cms), send to my own error “necessary data rewind wasn’t possible” and after see my request in HttpAnalyzer application I find out unity3d cant accept get cookie string . How can get cookie from web after login or register users ? my unity3d code :

var form = new WWWForm();
form.AddField( "SN", SystemInfo.deviceUniqueIdentifier.ToString());
form.AddField( "UserName ", UserName );
form.AddField( "year",Year );
form.AddField( "month", Month );
form.AddField( "day", Day );
form.AddField( "dob",Year+"/"+Month+"/"+Day );
form.AddField( "gender", Gender );
form.AddField( "password", Pass );
// Create a download object
var download = new WWW(URL, form);
// Wait until the download is done
yield download;
// show the Results
if(download.error != null)
    Debug.Log("Err :"+download.error);
    Flag = "1"; 

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