When running appfinder in collapsed view, the preferences button is hidden, which means you have to expand it and then click preferences in order to go to the 'custom actions' tab which only works in collapsed mode.
Patch?
Created attachment 9711 patch
Andre Miranda referenced this bugreport in commit 41ea75c8834652c0a7095c19a5d6c4d4c19183c7 Make Preferences button always visible (Bug #16489) https://git.xfce.org/xfce/xfce4-appfinder/commit?id=41ea75c8834652c0a7095c19a5d6c4d4c19183c7