Hold pointer in left corner of scrabble board until it changes to line, hold down left mouse button and drag to highlight the whole board, then hit Ctrl key and letter C which copies the board to the clipboard (or use whatever other method you prefer).
Now click on Reply. When reply is ready, hit Ctrl key and letter V which pastes the board into your reply. Type in the letters in the squares you want to use, one at a time, until your word is filled in. Then click on Send Message.
Be sure you are copying and pasting the last grid on the thread.