function onSubmit() {
    var instanceName = "something";
    var incidentURL = 'https://' + instanceName + '.service-now.com/incident.do?sys_id=' + g_form.getUniqueValue();
    copyToClipboardCustom(incidentURL);
}

function copyToClipboardCustom(text) {
  var input = document.createElement('input');
  input.style.position = 'fixed';
  input.style.opacity = 0;
  input.value = text;
  document.body.appendChild(input);
  input.select();
  document.execCommand('Copy');
  document.body.removeChild(input);
}