A new limited-time Pokémon Secret Club members only item is available to club members to celebrate that 150,000 people have pre-ordered the games and have joined the club. To commemorate this, all members of the Pokémon Secret Club will receive a serial code that will allow them to receive a Pokémon Quest Kakukoro T-Shirt that can be used in Pokémon Sword and Shield. Club members can get the serial code from November 1st, 2019 until January 31st, 2020. The code can used from November 15th, 2019 until February, 3rd, 2020.

Source: https://www.pokemon.co.jp/info/2019/10/191025_gm02.html