When I try this

fansh>WebClient(`http://en.wikipedia.org/wiki/C%2B%2B`).getStr

I get a 403 error. Any suggestions?

also..

fansh>WebClient(Uri.decode("http://en.wikipedia.org/wiki/C%2B%2B")).getStr 

doesn't work either.

Fantom Uri literals aren't expressed in encoded form. See http://fantom.org/doc/sys/Uri.html class header docs for full description. In your case you would need to decode that from Uri.decode