How can you find WhatsApp Username or ID?

Your ID/Username is your phone number or any other mobile phone number that you registered WhatsApp with.

Go to your settings on the right, click on your photo or on that line, on the very bottom you will see the number you are registered with.

When you register, you your phone number is set as your username by default. But you can change it again Go to Settings then click on your profile info. Right after your image you will see your username.