I believe something like this should work within a complex expression on a transformation rule (did not test). The dimension name logic may need some tinkering and I am not sure about the order of the base members being returned. If that's an issue you could tag the base member you want within a Text field and then include a where clause in the member filter so it will only return the member you want data loaded to. If no base members are returned it defaults to the source value. You might also need error handling if the source value isn't a valid member.
Dim DimensionToken As String = "E#"
Dim ParentMem As String = args.GetSource(DimensionToken)
If String.IsNullOrEmpty(ParentMem) Then Return Nothing
' Hardcode or replace with your logic to determine the dimension name
Dim DimensionName As String = "Entity" ' <<< set appropriately
' Get base members under the parent (metadata order)
Dim BaseMems As List(Of MemberInfo) =
BRApi.Finance.Metadata.GetMembersUsingFilter( _
si, DimensionName, DimensionToken & ParentMem & ".base", True)
If BaseMems IsNot Nothing AndAlso BaseMems.Count > 0 Then
For Each mi As MemberInfo In BaseMems
If mi IsNot Nothing AndAlso mi.Member IsNot Nothing Then
' Return the first base member name
Return mi.Member.Name
End If
Next
End If
' If no base descendants, return the parent itself
Return ParentMem
