User Configuration > Administrative Templates > Microsoft Office 2016 > File Open/Save dialog box > Restricted Browsing
User
HKCU\software\policies\microsoft\office\16.0\common\open find\restrictedbrowse\optin
When Restricted Browsing is activated the save as dialog box will be restricted such that the user will only be able to navigate to those locations and the children of those locations specified in the "Restricted Browsing\Approve Locations" policy setting. If you want to enable the "Approve Locations" policy setting, you must first enable the "Approve Locations" policy setting first.