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 configure whether Office displays a link to get a certificate from a Microsoft partner when there are no usable signing certificates. If you enable this policy setting, the link won’t be displayed. If you disable or don’t configure this policy setting, the link is displayed.