As you are considering a URL that will be used as such for a request (with HttpClient.getAsync) -- not as an argument within another URL -- you should use Uri.EscapeUriString.
Here is a comparison of three methods for the following URL:
var url = "http://some url?data=x y+z&user=1#ok";
Console.WriteLine(HttpUtility.UrlEncode(url));
http%3a%2f%2fsome+url%3fdata%3dx+y%2bz%26user%3d1%23ok
Obviously, this is not desired: the URL got damaged with / escaped, a + entered in the path, ...etc. The method seems useful for the query part of the URL, but not for the whole lot.
Console.WriteLine(HttpUtility.UrlPathEncode(url));
http://some%20url?data=x y+z&user=1#ok
This looks useful, although the space is a bit of a problem in the query part (notice the broken hyperlinking here, although browser can deal with it). But more importantly, the method is being deprecated:
Do not use; intended only for browser compatibility. Use UrlEncode.
Console.WriteLine(Uri.EscapeUriString(url));
http://some%20url?data=x%20y+z&user=1#ok
This seems to do the job well: %20 is an escape sequence that all modern browsers should support, also when occurring in the query part of the URL.