Nikon Z 8 Reference Manual page 157

2
Select a preset.
Hold the U button and rotate the sub-command dial until the desired white balance preset
(d-1 to d-6) is displayed.
3
Select direct measurement mode.
Release the U button briefly and then press it again
to enter direct measurement mode. L will flash in the
shooting display.
A white balance target (r) will appear in the center of the
frame.
157
White Balance