then we need first a good way to test if the prefix is set or not
you have a lot of prefixes (http:// , ftp:// but also mailto: (see other post of you))
Well you have to make sure the URL is in the valid URL format as defined by the w3c http://www.w3.org/Addressing/URL/Overview.html
As Johan stated we don’y know which protocol you want to use, mailto: is also possible.
If you let your users fillin http urls you have to make sure/validate they are prefixed with http://