This is more of a reminder for me than you that the DimensionDefaultingService class doesn't exist in D365.  The DimensionDefaultingService was a short-lived class; brought into existence for only one version of AX.  Its legacy lives on now in the LedgerDimensionFacade, where we can [most importantly] call the serviceCreateLedgerDimension() method to combine the RecId values of a LedgerDimension and up to four DefaultDimensions.  Of course it has a lot more to offer, but you'll have to look at the class code for that.