The official Pokémon website has revealed the code to get one of six exclusive Pikachu. The code is: PIKACHU20 but it can only be used once.

Pikachu Dons a Classic Chapeau

This exclusive Pokémon distribution grants Trainers a Pikachu wearing Ash’s original hat.

With the 20th Pokémon film, Pokémon the Movie: I Choose You!, making a limited theatrical run, we're celebrating by giving Trainers the opportunity to receive one of six exclusive Pikachu into their copy of Pokémon Sun or Pokémon Moon. These Pikachu are unique as they are wearing one of Ash's hats—each one reflecting the hat Ash wore in the various regions that the famous duo has explored in the twenty years of Pokémon TV and movies.

The first of these Pikachu is wearing a hat in the style of Ash's first adventures in the Kanto and Johto regions. Ash's simple and iconic original hat is instantly recognizable by Pokémon fans, and looks perfect on Pikachu, too. Enter your code by September 25, 2017, to get a Pikachu wearing this hat.

In addition to wearing Ash's original hat, this Pikachu is equipped with a set of moves inspired by the attacks it would frequently use in the Kanto and Johto regions. This Pikachu also, amusingly, comes inside a Poké Ball, a nod to the one that the Pokémon constantly refuses to travel within in the animated adventures.

PLEASE NOTE: Although there are six different behatted Pikachu to choose from, you will only be able to get one of them via this special code. The Pikachu you get will vary depending on when you enter this code. To receive Pikachu wearing Ash's original hat from the Kanto and Johto regions, you must enter the code between 0:00 UTC on September 19 and 23:59 UTC on September 25, 2017.

To get original hat Pikachu via a special code:

• Open your Pokémon Sun or Pokémon Moon game.
• Select Mystery Gift on the main menu.
• Select Receive Gift.
• Select Get with Code/Password, then Yes, then Yes again to connect to the internet.
• Enter the code PIKACHU20.
• Watch as you receive your Pokémon.
• Speak to the delivery person in any Pokémon Center to receive your Pokémon.
• Be sure to save your game.

Pikachu Details:

Level: 1
Nature: Hardy
Gender: Male
Ability: Static
Moves: Thunderbolt, Quick Attack, Thunder, Agility

Source: https://www.pokemon.com/us/pokemon-news/pikachu-dons-a-classic-chapeau/