just to be clear, are you talking about where you post just a link, and you want the embellishments added to it? or you mean you want something looking over your shoulder as you type, and every time you press <enter> it's supposed to inspect what you typed, looking for a url?

If it's the former, you can make an alias like

alias urlfix { if (#* iswm $active) msg $chan $1 <<<whatver>>> }

then you can just do:

/urlfix www.foobar.com

If it's the latter, that's a little more complicated, as it would need to parse your message for a url, and the definition for that can be ambiguous.