User Configuration > Administrative Templates > Microsoft Office 2016 > Privacy > Trust Center
User
HKCU\software\policies\microsoft\office\16.0\registration
This policy setting allows you to control whether the file format dialog has already been shown to the user. If you enable this policy setting, the dialog won't be shown again. If you disable this policy setting, the dialog prompts the user to select a default file format on each boot until one is selected. If you don't configure this policy setting, the dialog prompts the user to select a default file format on each boot until one is selected.