You can find your username on WhatsApp easily.
IPhone: Open your WhatsApp. Tap on ‘settings’ at the right corner. On the top, you can see your profile picture. Tap on it and you will see your username on the top.
On Android: Open your WhatsApp. Tap on three bars at the top right corner. Tap on ‘settings’ next and tap on the profile picture. Here you will find your username.
If you want to see the username of other number then go to his profile, you can read username below the profile picture. You can see it if the user has set the privacy of username to publicly.