There is NO nickname box at the top of the
Preferences window...
What you are showing in the screenshot is the "auto-coloration" feature. To edit your
nickname, use
/xcolornick command, not /xcolornickcfg
. To color your
text, use
/xcolortext command.
Auto-coloration, when enabled (see the checkbox next to tab's title - in
Preferences), colors the text provided as argument to /xcolornick with a default color or gradient, without showing the usual dialog (the one you're looking for).
For example, you enable Auto-coloration for Text, close the Preferences window, and type /xcolortext My Messsage. This message will be colored with a gradient effect from blue to red for example.