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