JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post">
  <input type="text" placeholder="Your name" autofocus required pattern="[a-z]{1,15}" />
  <input type="text" placeholder="Phone-number" required pattern="[0-9]{6,15}" />
  <input type="email" placeholder="Email" required/>
  <input class="button" type="submit" rel="reg_form" value="request call" />
</form>
<div class="popup">
  popup
</div>

CSS

.popup {
  display: none;
}

JavaScript

$('form').submit(function(e) {
  var empty = $(this).parent().find("input").filter(function() {
    return this.value === "";
  });
  if (!empty.length) {
    //Если все графы заполнены, то показываем popup
    $('.popup').show();
    //очищаем все данные текстовых полей, кроме кнопок
    $('form input').not(':button, :submit').val('');
  }
  e.preventDefault();
});