How to define composite primary key on code first

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VB

Anyone, please help me to define composite primary key on code first, it cannot to define both property code as below

Public Class TaskMapper
    Overridable Property TaskType As TaskType
    Overridable Property Task As Task
End Class

Public Class TaskType
    Inherits MasterData
End Class

Public Class Task
    Inherits MasterData
End Class

Public MustInherit Class MasterData
    Public Property Code() As String
    Public Property Name() As String
    Public Property Status() As String
End Class

Protected Overrides Sub OnModelCreating(ByVal modelBuilder As DbModelBuilder)
     'on this command raised an error  
modelBuilder.Entity(Of TaskMapper).HasKey(Function(tm) New With {tm.Task.Code, tm.TaskType.Code})
End Sub

What I have tried:

Able to define composite primary key