Answer by Grant for Check if checkbox is checked with jQuery

Since it's mid 2019 and jQuery sometimes takes a backseat to things like VueJS, React etc. Here's a pure vanilla Javascript onload listener option:

<script>
  // Replace 'admincheckbox' both variable and ID with whatever suits.

  window.onload = function() {
    const admincheckbox = document.getElementById("admincheckbox");
    admincheckbox.addEventListener('click', function() {
      if(admincheckbox.checked){
        alert('Checked');
      } else {
        alert('Unchecked');
      }
    });
  }
</script>