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I have never used it but SDK says:
VIII.3.2 SendMessage
C++
HRESULT SendMessage(long _msg_id, long _w_param, long _l_param);
C#
public void SendMessage(long _msg_id, long _w_param, long _l_param);
Visual Basic
Public Sub SendMessage(_msg_id As long, _w_param As long, _l_param As long)
Description
The function allows to send a message to the window.