Show Captcha by two step.
You should include js file from us. You also should have jquery already.
<script src="https://code.jquery.com/jquery-1.12.0.min.js" ></script>
<script src="http://www.captchaclick.com/js/jsclient.js" ></script>
Add code at the position where you want captcha showing.
<img class="initdata_picture_verification" />
It should be done. You can refresh your page to view the Captcha.
After that you may want to check the verification result of user's input. You can check it here