Steal trainers pokemon:
010156D0
Enter the above code as a GameShark code in your emulator's cheat menu.
Save your game before you activate the cheat in case anything goes wrong.
If you are playing on a GameBoy with the original Pokemon Yellow Special Pikachu Edition cartridge, then you will need an actual GameShark device to enter this or any GameShark code into the game.
Activate the code before the battle with the trainer starts then you should be able to fight and then try to catch his Pokemon with a Pokeball and keep it (as you would a Pokemon in the wild). Deactivate the code when done.
Only save after you have checked that you have the Pokemon, and not a bad egg or something!
We have more cheats and tips for this game here, Pokemon Yellow: Special Pikachu Edition cheats
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