User Configuration > Administrative Templates > Microsoft Excel 2016 > Excel Options > Security > Trust Center > External Content
User
HKCU\software\policies\microsoft\office\16.0\excel\security\external content
This policy setting allows you to control whether Dynamic Data Exchange (DDE) server lookup is allowed. By default, DDE server lookup is turned on, but users can turn off DDE server lookup by going to File > Options > Trust Center > Trust Center Settings > External Content. If you enable this policy setting, DDE server lookup isn’t allowed, and users can’t turn on DDE server lookup in the Trust Center. Note: If you’re using Dynamic Data Exchange (DDE) server launch, which isn’t recommended, don’t enable this policy setting, because DDE server launch requires DDE server lookup to be on. If you disable or don’t configure this policy setting, DDE server lookup is turned on, but users can turn off DDE server lookup in the Trust Center. Note: This policy setting only applies to subscription versions of Office, such as Microsoft 365 Apps for enterprise.