Laravel Spark - Adding additional fields to registration form, but when empty no errors are returned

I'm building my first app with Laravel 5.2 & Laravel Spark. The front end is built using Vue.js I believe and despite adding the following to register-common-form.blade.php:

<!-- Username -->
<div class="form-group" :class="{'has-error': registerForm.errors.has('username')}">
    <label class="col-md-4 control-label">Username</label>

    <div class="col-md-6">
        <input type="name" class="form-control" name="username" v-model="registerForm.username" autofocus>

    <span class="help-block" v-show="registerForm.errors.has('username')">
        @
    </span>
    </div>
</div>

I can't actually see a way to fully register that extra field so that it is picked up for error handling. I've got it so that the UserRepository handles the field and inserts it, but just can't get the front end errors to show properly.

Is anyone able to help with this at all?

via Chebli Mohamed