function counter_summ() {
...
}
<form class="main-head__form" method="post" action="https://smartresponder.ru/subscribe.html" name="SR_form_383269_70">
<input name="field_email" type="text" placeholder="Введите Ваш e-mail" autofocus>
<input type="submit" name="subscribe" class="btn-blue">
</form>