SendMessage API in C# - Sending a mouse click to another window

i need to click on a button in another application from my C# application, i have API calls working to activate or close the window etc and they work fine so i know i am sending messages, and i know the code for an api mouse click, however i need to know the code to convert the integer mouse coordinates into the correct format so the correct area gets clicked in the other application.  Here is my sample code:

      public const uint WM_LBUTTONDOWN   = 0x0201;
      public const uint WM_LBUTTONUP     = 0x0202
[DllImport("user32.dll")]
            public static extern int SendMessage(
                  int hWnd,      // handle to destination window
                  uint Msg,       // message
                  long wParam,  // first message parameter
                  long lParam   // second message parameter
                  );
public static void SendMouseClick(int iHandle, int X, int Y)
            {
/***problem line***********      / long lparm=(X and Y converted coordinates)
                    long lngResult = SendMessage(iHandle, WM_LBUTTONDOWN, 0, lparm);
                    long lngResult2 = SendMessage(iHandle, WM_LBUTTONUP, 0, lparm);

            }

From my app i call

SendMouseClick(windowHandle,100,200)
i need to know how to figure out the correct value for the last parameter and pass it to the sendmessage function.
In some sample coordinates i saw online it stated that a Y value of 200 and an X value of 100 would look something like 0064 00C8 but i have no idea how to get to that format from 200 and 100 and how to put them together, thanks in advance for the help
Ruby