Actually, I am somewhat surprised with the original answer given. I am an International Arbiter, Category A, and have been arbiter at many top-level events and with many world-class players, including the current World Champion and two former World Champions.
Here are the applicable sections of the FIDE Laws of Chess:
Article 6.2.1 - "During the game each player, having made his move on the chessboard, shall stop his own clock and start his opponent’s clock (that is to say, he shall press his clock). This “completes” the move."
Article 6.6 - "At the time determined for the start of the game White’s clock is started."
Article 7.5.3 (probably the most applicable and important in this situation) - "If the player presses the clock without making a move, it shall be considered and penalized as if an illegal move."
Article 8.1.2 (also applicable and important in this situation) - "It is forbidden to record the moves in advance, unless the player is claiming a draw according to Article 9.2, or 9.3 or adjourning a game according to Guidelines I.1.1"
I understand in these times when potential cheating is something that we need to consider, so it is up to the organizer, if the games are being transmitted, to take the necessary steps to incude delaying the transmission. That said, nowhere in the FIDE Laws of Chess is there a rule stating that a player may just write the move and later make the move when the opponent shows up.