D*N, first, you need to have Wi-Fi Connection.
You also need to have Mystery Gift (you talk to a person in Jubilife city and he gives you it this is how to do it)
Go to Jubilife city and go to the third (I think I don't remember) and talk to a man in there there will be one who will ask you for an interview, in the first interview say these words: EVERYONE HAPPY and then he will interview you again asking for feedback. This time you say: Wi-Fi CONNECTION. Save then turn off your DS and put it back on and in the menu screen when you first turn it on it will say MYSTERY GIFT
You do not have to go to Gamestop for this, I got it from my own home.
After you clicked Mystery Gift, click via wifi connection and you will see ;172;
After you got it, Go to ANY Pokemart and there will be a man, talk to him and he will give you the Pichu (if your party is full then you will have to leave one Pokemon in your PC Box) then SAVE
done.
EDIT: This event just works on Diamond, Platinum and Pearl versions