laravel with errors custom code example

Example 1: $errors show this error in laravel

<input type="text" name="firstname">
@if($errors->has('firstname'))
    <div class="error">{{ $errors->first('firstname') }}</div>
@endif

Example 2: how to add an custom error to validater error in laravel

if (request('event') == null) {
    $validator->errors()->add('event', 'Please select an event');
}