Bogdan Sokolovskyi

<form id="myForm">
    Имя:
    <input type="text" name="name">
    <input type="submit" value="Submit">
  </form>

  <script>
    let form = document.getElementById('myForm');
    form.addEventListener('submit', validateForm);

    function validateForm(e) {
      var x = form.elements.name.value;
      if (x == null || x == "") {
        form.elements.name.value = "Необходимо заполнить поле Имя!";
        e.preventDefault();
      }
    }
  </script>