You can see it in every pages url, in your browsers url field.
Beyond that, don’t waste your breath. IT WILL NEVER BE DISPLAYED ON THE PAGE in kiwitrees.
I hope that is clear enough. If you want to understand the reasons go read the pages and pages about it on the webtrees forum archives. This is a dead subject.
Sorry to be blunt, but if it’s that important to you then you will need to find a different family history product, if you can find one that does actually display it.
For the record, REFN is something completely different. You are referring to the “xref”, a random machine generated 11-digit code for internal database use ONLY.
There is a code called REFN that you can manually create and manage that you can use, and a couple of others (read the GEDCOM spec). They can be displayed by kiwitrees.