cssd-back/app/Http/Requests/StoreTransferRequest.php

<?php

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;

class StoreTransferRequest extends FormRequest
{
    public function authorize()
    {
        return true;
    }

    public function rules()
    {
        return [
            'user_id' => 'required|exists:users,id'
        ];
    }

}