How to whitelist some characters using HTML sanitizer C#

I am using HTML sanitizer to detect tag or script in input but I want to allow some characters in input like &, *. However, the sanitizer method replace the character & to & which is wrong

What I have tried:

public static bool IsDangerousString(string raw)
    {
        // Using HTML sanitizer nuget for more info https://github.com/mganss/HtmlSanitizer
        raw = abc&123;
        var sanitiser = new HtmlSanitizer();
        var sanitised = sanitiser.Sanitize(raw);
        return raw != sanitised; // should return false but returns true
    }