SCOPE function for tabular model

Hi,

I used to have multidimensional SSAS cube where I have parent-child hierarchy and aggregations of children members should be substituted with value for parent coming from source system. For these purposes I use MDX expressions:

SCOPE(MeasureGroupMeasures("MeasureGroup"))

THIS = [Dim].[Hierarchy].CURRENTMEMBER.DATAMEMBER

END SCOPE

I'd like to replicate the cube as tabular mode but I am having difficulties with the expression in DAX instead of SCOPE expression in MDX.

Basically, I have children A= 5, B = 3, C = 7 and by default they will be summarized for parent as D = A + B + C = 15.

I need to have a value from source system for parent instead of aggregation as D = 12 for example.