Thank You, made suggestion for that parameter and text field for custom parameters on that page...
 

-----Ursprüngliche Nachricht-----
Von: Lee <ler762@gmail.com>
Gesendet: Montag, 28. Oktober 2019 22:41
An: "Ingo Wolf" <ingo.wolf@gmx.de>
Cc: html-tidy@w3.org
Betreff: Re: keep &uuml; &auml; &ouml; ...
</ingo.wolf@gmx.de>

On 10/27/19, Ingo Wolf <ingo.wolf@gmx.de> wrote:
> Hi, I'v tried every Char encoding on
>
> https://infohound.net/tidy/
>
> can't get &uuml; &auml; &ouml; ... kept in output,
>
> what do I have to set

--preserve-entities yes

$ tidy -q --preserve-entities yes --tidy-mark no test.html
line 3 column 1 - Warning: inserting missing 'title' element
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<p>this is &amp;uuml;&nbsp; &uuml;</p>
<p>this is &amp;auml;&nbsp; &auml;</p>
<p>this is &amp;ouml;&nbsp; &ouml;</p>
</body>
</html>

Regards,
Lee