How to reauired

How to creare if you choose marketing, you must enter the code

<div class="form-group row d-none" id="referral_code">
  <label for="referral_code">Referral Code</label>
  <div class="col-md-10">
    <input id="referral_code" type="text" class="form-control" name="referral_code">
  </div>
</div>
<input class="form-check-input" type="radio" name="familiar" id="google" value="1" onclick="showReferal(false)">
<input class="form-check-input" type="radio" name="familiar" id="friends" value="2" onclick="showReferal(false)">
<input class="form-check-input" type="radio" name="familiar" id="advertising" value="3" onclick="showReferal(false)">
<input class="form-check-input" type="radio" name="familiar" id="marketer" value="4" onclick="showReferal(true)">

MyRequest.php

<?php 

namespace App\Http\Requests;

use App\Http\Requests\Request;

class MyRequest extends Request {

    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
            if (familiar == 4) {
                'referral_code' => 'required',
            }
        ];
    }

}