You can close a window by sending it the WM_CLOSE command.
To send the message to the window, use SendMessage like what is used to send messages to other windows used in the Command5 button.
Call SendMessageLong(Handle of Window to Close, WM_CLOSE, 0, 0)
Notice, that the call requires the handle of the window to be closed. So, get the handle using FindWindow. FindWindow requires the windows Class Name and its Title in order to return the handel. We already know the class name from other code in the example and the windows title.