In Red or Blue:
018575D1 will put it as the third move in Red and Blue or the fourth move in yellow.
(Keep shark switch off, put Mew as first position monster, turn switch on, then turn switch off. Check Mew to make sure it worked correctly. If it did, save the game. If it didn't do *not* save the game)
Keep in mind that a Mew with Amnesia isn't even remotely tournament legal, and I sure as heck wouldn't play against one for real!
If you have any problems, feel free to ask.
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-Fortune Cookie Wisdom