Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 130)

2.
Select a custom white balance number to be registered.
Press < >.
Turn the < > dial to select a number from 1 to 5 for < * >, then
press < >. The custom white balance will be registered to the
selected number.
3.
Select [Record and register WB].
The screen now turns off, and the selected number blinks on the top
LCD panel.
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