/// <summary>
        /// This message is sent by an application to simulate the user clicking a button. This message causes the button to receive a WM_LBUTTONDOWN and a WM_LBUTTONUP message, and the button's parent window to receive a BN_CLICKED message.
        /// </summary>
        public enum ButtonControlMessages: uint
        {
            BM_GETCHECK = 0x00F0,
            BM_SETCHECK = 0x00F1,
            BM_GETSTATE = 0x00F2,
            BM_SETSTATE = 0x00F3,
            BM_SETSTYLE = 0x00F4,
            BM_CLICK    = 0x00F5,
            BM_GETIMAGE = 0x00F6,
            BM_SETIMAGE = 0x00F7
        }

Example

// buttonWindowHandle - handle of Button window

SendMessage(buttonWindowHandle, BM_CLICK, (IntPtr)0, (IntPtr)0);