Message box when in range....

Hello everyone,
I need your help.
I have a following VBA code:

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("B20:B1000")) Is Nothing Then
    Select Case Target.Value
        Case Is = "A", "B"
            MsgBox "Fill out" 
    End Select
End If
End Sub

My question is what should the code looks like (if possible) when to this I want that when in Range C20:C1000 I enter EHS a MsgBox pops up with "Agree?"

Thanks for your help