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 OPER Tool #5427 Master CU Parameter Config.  Online CUs please contact a CSR for assistance.