[roman]

Me he encontrado este ejemplo, lo he probado y funciona:

Código Delphi [-]
function VarArrayFromString(const S: String): Variant;
begin
  Result := Unassigned;

  if S <> '' then
  begin
    Result := VarArrayCreate([0, Length(S) - 1], varByte);
    Move(Pointer(S)^, VarArrayLock(Result)^, Length(S));
    VarArrayUnlock(Result);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  PostData: OleVariant;
  Headers: OleVariant;

begin
  PostData := VarArrayFromString('campo1=valor1&campo2=valor2');
  Headers := 'Content-Type: application/x-www-form-urlencoded'#13#10;

  Browser.Navigate('http://tu/pagina/web.html', EmptyParam, EmptyParam, PostData, Headers);
end;

// Saludos