From the help file, this is a bit tricky but it can be done:
You can now create a special alias called noresults.
If user hits enter when no results are shown, this alias will be triggered.
It can be useful if you want to do something like launch what they type directly (as if it was on path), or invoke another search program, etc. Just use $$1 in the results.
Important: you need to set the regular expression pattern to:
So if you wanted to do a web search your result would be just:http://www.google.com?
q=$$1 or http://www.google.com?
(the u1 is for better url encoding of the parameter)
or if you wanted to show the web search inside farr's window use result