BYTE pascal SetWindowByte(HWND hWnd, int iByte, BYTE byte)

  Sets the value of the single byte residing at the 'iByte' position
in the window extra area.

Parameters
  hWnd is the window handle of the window to query.
  iByte is the 0-based byte offset from the start of the window extra area.
  byte is the single byte value.

Returns
  The byte at pWinExtra[iByte].