This only works if you play on your mobile with an account registered on the Pixel portal.
If you're a mobile player and not playing in guest mode, you can check your player ID when you log in from within the Pixel Portal.
On your mobile, switch your browser to desktop mode and then open the Pixel portal.
Log in and start the game.
You don't have to wait for the game to load, as you will see your player ID at the bottom ( at loading 0%!!!).