You need Firefox and the Addon Greasemonkey or Scriptish.
You have to edit the Setting Block in the Script to reflect your NZBGet Settings (IP,Port,Password) and you can set a NZBGet Category and Priority for each Newznab Category.

The Link Name changes to OK or ERROR, depending if the transmission to NZBGet worked or not.
Note: OK only means the the URL has been successfully transmitted to NZBGet, not that the nzb has been download or added to the queue.
Tested with:
nzbs.org
newztown.co.za
nzb-matrix.eu
Not working with:
usenet-crawler.com This site uses Links are redirected to a Advertising Site.
Version 1.1
update for changed newztown.co.za
Version 1.2
Added custom category support
included custom categories for nzbs.org and newztown.co.za
Available from: https://userscripts.org/scripts/show/163279