Simply head over to any of Sudoku.com's Sudoku puzzles (specifically, any of "Easy", "Medium", "Hard", and "Expert"). Once there, you will notice that if you hold the shift key, the "Notes" icon (looks like a pencil, located either to the right or below the Sudoku puzzle) will indicate that "Notes" are now ON. As soon as you release the shift key, "Notes" will turn back OFF. You can invert this if you would like to take notes by default, by simply clicking on the "Notes" button and then holding shift to input an actual answer.