What do you mean "how do you enable that feature"?
By default the talkgroup ID will be displayed on
each trunked system transmission unless you program
an alpha tag in place of the ID.
And even if you program an alphatag you can still view the ID
on the 3rd display line.
Us veteran Uniden XT users don't mind helping out, but it might behoove you
to read over the
396XT Complete Reference as the answers to several of
your questions are answered in that Guide. Yes, it is a complete reference
guide for the 396XT, but considering that the 396XT and 996XT are almost
identical the guide covers both.
Marshall KE4ZNR