HTML:

<form id="emailfill">
  <input type="email" id="my-text-input" />
  <input type="submit" />
</form>

JS:

$('#emailfill').on('submit', function(event) {
  event.preventDefault();
  if ( $('#my-text input').val() == '') {
    alert('You missed the field.');
  } else {
    alert('Thanks for filling the field!');
  }
})