JSFiddle - React, Tailwind, and code Playground

HTML

<form id="contactform" action="" method="post">
  <input type="submit" name="submit">
</form>

<script>
let formulier = document.querySelector("#contactform");
formulier.addEventListener('submit', function(event) {
  if (confirm("Druk op OK als u het formulier wenst te versturen.") == false) {
    event.preventDefault();
  }
});
</script>