To tie in with the 20th movie, Pokémon the Movie: I Choose You!, another special promotion has begun in Japan. If you prebook tickets in Japan, you will get a serial code that can be used to unlock one of six Pikachu wearing one of Ash’s caps from over the years. The latest distribution provides Alola Cap Pikachu, which has Ash’s hat from the Sun & Moon series of the anime and knows the move Iron Tail, which Pikachu can’t normally get in the Sun and Moon games. This Pikachu also comes with the special Pikashunium Z Z-Crystal, which turns the move Thunderbolt into the Z-Move 10,000,000 Volt Thunderbolt. This distribution is available until July 17, 2017, with a repeat scheduled for September.
Source: Serebii
Pokémon Blog 
What is the serial code?
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Unfortunately, it’s only available in Japan at the moment
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