I read this question the 1st time and thought you were asking a (fairly basic) question about how the setting service works, but then I re-read and now not so sure that you don’t already know all that and are actually asking a more complicated question about how to use the default value regardless what the current setting service value is. As I’m not sure I will answer for both interpretations.
1st interpretation: How to set default values in the setting service
Global default values for key point of view hierarchies are defined in the contentStore cube }APQ UX Settings Service Dimensions. To do this at a minimum the dimension names for the key point of view dimensions for each instance need to be defined in the master list dimension }APQ UX Dimension. You can just manually edit the dimension and copy/paste the dimension names in using Arc and then manually populate the “IsUsed” and “IsUsedSettingService” flags and the default member value. Or you can set up the metadata syncing service to automate this. If using the metadata syncing then the only manual step you need to do as an administrator is to set the “IsUsedSettingService” flags. (You only really should do this for the key PoV dimensions which will be used in the filter a lot of the time).
Since you gave a link to the documentation then you already noticed that online documentation for the metadata syncing is something we are lagging behind on! The syncing service is done by a (Windows) command line execuitable which is compiled from a tm1py script. Here’s a link to the latest version. The zip contains the exe and a PDF with some pretty solid documentation of the parameters and options which can be used.
Once global default members have been set in the }APQ UX Settings Service Dimensions cube this automatically flows via rule into the }APQ UX Settings Service UserPreference cube. This is the cube that gets written back to via the provided “User Default Settings” UI. If you know in advance about individual users requiring different default values then you could preset these in the Settings Service User Preference cube. Otherwise just leave it up to the users to do it themselves.
On login UX will check in Settings Service User Preference cube and set the initial setting service value based on what is configured there. For any dashboard or view to use the setting service value what you need to do for any filter or fixed dimension is leave the “selected element” value blank.
Both @wwang and @crashedi know all of the above inside out and can help you with the setup if you have any issues or questions.
2nd interprretation: how to use the default value rather than the current setting service value
OK. Suppose you already knew all of the above and the real question is “regardless what the current settings service value is how could you make the filter value in a specific dashbord ALWAYS be what the user’s default value is?”