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<form enctype="multipart/form-data" action="https://steamcommunity.com/" method="POST" name="loginForm" id="loginForm">
  <input type="hidden" name="action" value="doLogin" />
  <input type="hidden" name="goto" value="" />
  <label for="steamAccountName">Steam Username:</label>
  <input class="textField" type="text" name="steamAccountName" id="steamAccountName" />
  <label for="steamPassword">Password:</label>
  <input class="textField" type="password" name="steamPassword" id="steamPassword" autocomplete="off" />
  <input class="subImage" type="image" id="imageLogin" onMouseover="iSwap('imageLogin','skin_1/buttonLoginOver.gif')" onMouseout="iSwap('imageLogin','skin_1/buttonLogin.gif')" src="https://steamcommunity.com/public/images/skin_1/buttonLogin.gif" width="104" height="25" border="0"/>
</form>                    

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<div class="inputFeld">
<p class="lgbox" style="margin-left: 5px !important; margin-left: 3px; "><b>Benutzernummer:</b></p>
<input type="text" name="Login" value="" maxlength="50" style="width:140px;" size="14">
<input type = "hidden" name = "Input_Login_Mandatory" value = "Yes">
<div style="height:1px;font-size:1px; clear:both"></div>
<p class="lgbox" style="margin-left: 5px !important; margin-left: 3px; "><b>Passwort:</b></p>
<input type="password" name="Password" maxlength="50" style="width:140px;font-size=12px;" size="14">
<input type = "hidden" name = "Input_Password_Mandatory" value = "Yes">
<div style="height:1px;font-size:1px; clear:both"></div>
<input type="image" src="https://www.testseite.de/login.gif">
</div>                    

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<form name="LoginForm" action="https://www.testseite.de/de_DE/ViewLogin-Process?Next=" method="POST">
<input type="hidden" name="NextPipeline" value="">                    

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https://www.testseite.de/de_DE/ViewLogin-Process?Next=&Input_Login_Mandatory=Test&Input_Password_Mandatory=Testpass                    

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var
  Params : TStringList;
  HTMLCode : string;
...
  Params := TStringList.Create;
  Params.Add('action=doLogin');
  Params.Add('goto=');
  Params.Add('steamAccountName=' + Memo1.Lines[i]);
  Params.Add('steamPassword=' + Memo2.Lines[i]);
...
  HTMLCode:=IdHTTP1.Post('https://steamcommunity.com/', Params);                    

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//nach IdHTTP1.Post()
HTMLCode:=utf8decode(HTMLCode);                    

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DeineStringList.text :=                    

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  Params:=TStringlist.create;
  Params.Add('action=doLogin');
  Params.Add('goto=');
  Params.Add('steamAccountName=' + Memo1.Lines[i]);
  Params.Add('steamPassword=' + Memo2.Lines[i]);
  Params.Add('x=35');
  Params.Add('y=14');

  IdHTTP1.Request.Referer := 'https://steamcommunity.com/?action=doLogout';
  IdHTTP1.Request.Host := 'steamcommunity.com';
  IdHTTP1.Request.ContentType := 'multipart/form-data;';
  IdHTTP1.Request.Accept := 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8';
  IdHTTP1.Request.AcceptCharSet := 'ISO-8859-1,utf-8;q=0.7,*;q=0.7';
  IdHTTP1.Request.AcceptEncoding := 'gzip,deflate';
  IdHTTP1.Request.AcceptLanguage := 'de-de,de;q=0.8,en-us;q=0.5,en;q=0.3';

  HTMLCode:=IdHTTP1.Post('https://steamcommunity.com/', Params);                    

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  //globale Variable
  var
    HTMLCode: String;

  ...

var
  Params: TStrings;
begin
  Params := TStringlist.Create;
  try
    Params.Add('action=doLogin');
    Params.Add('goto=');
    Params.Add('steamAccountName=' + EditAccount.Text);
    Params.Add('steamPassword=' + EditPassword.Text);
    Params.Add('x=35');
    Params.Add('y=14');

    IdHTTP1.Request.Host := 'steamcommunity.com';
    IdHTTP1.Request.UserAgent := 'Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.1) Gecko/20090624 Firefox/3.5 (.NET CLR 3.5.30729)';

    try
      HTMLCode := IdHTTP1.Post('https://steamcommunity.com/', Params); //evtl. noch ein Dokument (index.php?) angeben... 
    except
      on E: Exception do
        ShowMessage(E.ToString + sLineBreak + E.Message);
    end;
  finally
    Params.Free;
  end;
end;                    

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  IdHTTP1.Request.ContentType := 'multipart/form-data;';                    

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IdHTTP1.Request.Referer := 'https://steamcommunity.com/?action=doLogout';                    

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    IdHTTP1.Request.Accept := 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8';
    IdHTTP1.Request.AcceptCharSet := 'ISO-8859-1,utf-8;q=0.7,*;q=0.7';
    IdHTTP1.Request.AcceptEncoding := 'gzip,deflate';
    IdHTTP1.Request.AcceptLanguage := 'de-de,de;q=0.8,en-us;q=0.5,en;q=0.3';                    

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procedure TForm1.Button1Click(Sender: TObject);
var
  HTMLCode : String;
  Params : TStringlist;
begin
  Params := TStringlist.Create;
  try
    Params.Add('action=doLogin');
    Params.Add('goto=');
    Params.Add('steamAccountName=' + Edit1.Text);
    Params.Add('steamPassword=' + Edit2.Text);
    Params.Add('x=35');
    Params.Add('y=14');

    IdHTTP1.Request.Referer := 'steamcommunity.com/index.php';
    IdHTTP1.Request.Host := 'steamcommunity.com';                       //contenttype NICHT auf multipart stellen, sonst geht es nicht!
    IdHTTP1.Request.UserAgent := 'Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.1) Gecko/20090624 Firefox/3.5 (.NET CLR 3.5.30729)';

    try
      HTMLCode := IdHTTP1.Post('https://steamcommunity.com/', Params);
    except
      on E: Exception do
      begin
        if E.Message='<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">' then
        begin
          URL:=Stringreplace(URL, ':443', '', [rfReplaceAll]);
          HTMLCode:=IdHTTP1.Get(URL);
          Memo2.Text:=Memo2.Text + HTMLCode;
        end
        else
          ShowMessage('Login fehlgeschlagen: "' + E.Message + '"');
      end;
    end;
  finally
    Params.Free;
  end;
end;

...

procedure TForm1.IdHTTP1Redirect(Sender: TObject; var dest: String;
  var NumRedirect: Integer; var Handled: Boolean;
  var VMethod: TIdHTTPMethod);
begin
  if (VMethod = hmPost) then VMethod :=hmGet;
  memo1.Lines.Add('Redirect:');
  memo1.Lines.Add('  Dest: '+dest);
  URL:=dest;
  memo1.Lines.Add('  NumRedirect: '+IntToStr(NumRedirect));
  memo1.Lines.Add('  Handled: '+IntToStr(Integer(Handled)));
  memo1.Lines.Add('  Method: '+IntToStr(Integer(VMethod)));
  memo1.Lines.Add('');
end;