How to enable custom fields for the User?

link not working

This thread is from almost 6 years ago.  The link in question goes to an old thread on Appian Forum (in Tempo), and now Tempo access has been turned off, so it's hard to guess what it may have been pointing to.  If you can tell us more about what you need / what's not working, we can probably point you to some more up-to-date information.  To be slightly more proactive, I'll point out that the People Functions plug-in has a smart service that allows the Custom fields to be set.

Tell us about your use case.