Declare Auto Function FindWindow Lib "USER32.DLL" (ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
  Declare Auto Function SetForegroundWindow Lib "USER32.DLL" (ByVal hWnd As IntPtr) As Boolean


  Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    Dim calculatorHandle As IntPtr = FindWindow("CalcFrame", "Калькулятор")'в этом месте может потребоваться изменение, в зависимости от системы

    If calculatorHandle = IntPtr.Zero Then
      MsgBox("Калькулятор не запущен.")
      Return
    End If

    SetForegroundWindow(calculatorHandle)
    SendKeys.SendWait("123")
    SendKeys.SendWait("+")
    SendKeys.SendWait("456")
    SendKeys.SendWait("=")
  End Sub