Using HttpUtility requires a reference to the System.Web library:

@ref 'System.Web'
use System.Web

class Test

    def main
        text = 'foo&bar'
        encoded = HttpUtility.urlEncode(text)
        assert encoded == 'foo%26bar'
        assert HttpUtility.urlDecode(encoded) == text

See also: LibraryTopics, WebDevelopment