When using Creole for markup, creating an external link appears to be impossible. Neither \[[Outside URL|http://example.com]] nor <<http://example.com>> nor \[Outside URL]\(http://example.com) work. The first gets rendered as a broken WikiLink, the second get eaten and the last is not parsed in anyway so you end up with that exact text in your page.
I'd have made this as a Creole page as a practical demonstration, but that doesn't seem possible here. Here's a page with an example: <https://www.icanttype.org//demo/CreoleExternalLinks>
+
+Looking at my example it seems that this is now [[done]] at least in 3.20100815.7 (Debian)