Use Tool #15 "Update Membership Information."  On the third (final) screen, check the "Dividend withholding" checkbox.  (Refer to the link to online help below for a sample of this screen.)

NOTE: The % of dividends that is withheld is controlled by a setting in CU Master Parameters.  Self Processors can access this via MNOP09 #1.  Online CUs please contact a CSR for assistance.