Just a quick note on adding javascript to your OctoberCMS site.
You can do it inside the backend view as follows :-
{% put scripts %}
<script src='https://www.google.com/recaptcha/api.js'></script>
{% endput %}
If you have some code to run in a custom plugin :-
public function onRun() {
$this->addJs('https://www.google.com/recaptcha/api.js');
}