There have never been any codes for any of the core Pokemon titles.
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You can't the only possible way to get rayquaza in platinum is to transfer it from emerald, get it at GTS (you must have seen a rayquaza to get it at GTS) or use AR (Action Replay). hope this helped.
How do you catch rayquaza in Pokemon black and white
You could look for the Pokemon modifier code (for the code to work, you need the national pokedex) then, type in rayquaza's pokedex number in the poketch's calculator, press L or R i think, and voilla, u've got yourself a rayquaza.
Pokemon Emerald version: Master Code: D8BAE4D9 4864DCE5 A86CDBA5 19BA49B3 Wild Pokemon Modifier with Rayquaza (It requires the Master Code listed above): B749822B CE9BFAC1 A86CDBA5 19BA49B3 Rayquaza in the wild: F5E36E5E CFCDF0F0
There is no such code for pokemon platinum