reverse proxy to external url

No iRule required. If you set up a virtual server on your internal interface and connect it to a pool with the pool member configured as FQDN, you can enter your url and any request will automatically be sent out to that specific url. It will even persist the uri of the original request. So you should just be able to make a virtual server attached to a pool with an FQDN pool member attached to the domain name of wherever you would like it to go.