User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings > Digital Signatures
User
HKCU\software\policies\microsoft\office\16.0\common\signatures
This policy setting allows you to set the HTTP URL for the timestamp server used by Office 2016 applications in the process of validating messages or documents. If you enable this policy setting, you must provide a valid HTTP URL address for the timestamp server. If you do disable or not configure this policy setting, a timestamp server will not be used.