How to URL encode strings in C#

Uri.EscapeDataString does what you want. See MSDN.

According to RFC 1738:

Thus, only alphanumerics, the special characters "$-_.+!*'(),", and
reserved characters used for their reserved purposes may be used
unencoded within a URL.

Neither HttpUtility.UrlEncode nor WebUtility.UrlEncode will encode those characters since the standard says the parentheses () can be used unencoded.

I don't know why the URL Encoder / Decoder you linked encodes them since it also lists them as as a character that can be used in a URL.