First of all, you need to add the username field in the users table. To do that, add the following in create_users_table.php

	$table->string('username');

Now, migrate it.

php artisan migrate:refresh

Add it in the User.php model in the protected field:

protected $fillable = [
        'name', 'email', 'password', 'username',
    ];

Now, we have to add it in the RegisterController.php

    protected function validator(array $data)
    {
        return Validator::make($data, [
            'username' => ['required','string','max:255','unique:users','alpha_dash'],
        ]);
    }

    /**
     * Create a new user instance after a valid registration.
     *
     * @param  array  $data
     * @return \App\User
     */
    protected function create(array $data)
    {
        return User::create([
            'username' => $data['username'],
    }

Add it in the view:

<input type="text" name="username" placeholder="Username">

Now, you should be good to go!