I have Pokemon bank, and you don’t use it “in the game”. You start it from the menu screen, then select the game you want to use Pokémon bank with and where you want to move the Pokémon. Keep in mind that it can only be used with Gen 6 and 7, and the Poké transporter can be used with gen 5 and the virtual console games.
Source: My experience with Pokémon Bank.