This is a custom HTML tag for GTM that fills out a form field with the data provided by a variable

Form Fill Demo

<script>
(function () {
  var value = "{{cookie - __gtm_campaign_url}}"         //fill with variable which information should be written into the field
  var selector = "input[name='source']"      //fill with CSS Selector that identifies the form field
  var field = document.querySelector(selector)
  if(field){ field.value = value||"(none)"; }
})();
</script>

Form Fill Template

<script>
(function () {
  var value = ""         //fill with variable which information should be written into the field
  var selector = ""      //fill with CSS Selector that identifies the form field
  var field = document.querySelector(selector)
  if(field){ field.value = value||"(none)"; }
})();
</script>