请教OnClick = 怎么改成打开外部程序

请问各位大神，这样的代码 OnClick = bbExitClick怎么改成OnClick = 打开外部程序
 object bbExit: TImageButton
    Left = 49
    Top = 390
    Width = 54
    Height = 53
    Cursor = crHandPoint
    Shape = ibsRoundRect
    ResUp = 'ubExit'
    ResPoint = 'pbExit'
    ResDown = 'dbExit'
    PolyShift = 15
    Color = clGray
    ExternalLabel = False
    Font.Charset = DEFAULT_CHARSET
    Font.Color = clWhite
    Font.Height = -19
    Font.Name = 'PragmaticaCTT'
    Font.Style = [fsItalic]
    ParentFont = False
    TabOrder = 4
    OnClick = bbExitClick
  end