courtnEMAIL

MktoForms2.loadForm("//app-sjXX.marketo.com", "XXX-XXX-XXX", formID, function(form) {
  var myUTM = your_utm_function(); // This function is your custom logic to retrieve the user's UTM codes.
  form.setValues({
    "utm_source": myUTM.source;
    "utm_campaign": // etc etc
  });
});

courtnEMAIL

MktoForms2.loadForm("//app-XXXX.marketo.com", "XXX-XXX-XXX", formID, function(form) {
  form.setValues({
    // Update this with the field/value.
    "my_field": "my value!"
  }, function(k, v){var o={};o[k]=v;form.addHiddenFields(o)});
});