Tap the Settings button (gear icon) in the top right corner to open the Settings Menu.

Your Player ID will be shown under the Account tab.

Your Player ID is also shown on most loading screens.

For Nintendo Switch:

Tap on the Settings button (gear icon) in the top right corner to open the Settings Menu. Or simply press the plus (+) button on the right joy-con.

Use the L or R buttons to open the Account tab. Your Player ID will be displayed there.

You can also see your Player ID on the loading screen.