You can link an account from the menu or title screen of the game.
Please try the following steps.
Note: Players under 16 years old can only link a Nintendo Account.
Only iOS devices can link to an Apple Account.
From the menu in the game
1. Menu (rightmost button at the bottom of the screen) > Other > Account
2. Select the account you wish to link from the account screen.
3. Enter the login information of the account you selected.
From the title screen
1. Menu (top-right button on the screen) > Account
2. Select the account you wish to link from the account screen.
3. Enter the login information of the account you selected.