Grab company GET parameter.

company.js

<script type="text/javascript">
  
  $(function() {
  
  var getUrlParameter = function getUrlParameter(sParam) {
  var sPageURL = decodeURIComponent(window.location.search.substring(1)),
  sURLVariables = sPageURL.split('&'),
  sParameterName,
  i;
  
  for (i = 0; i < sURLVariables.length; i++) {
  sParameterName = sURLVariables[i].split('=');
  
  if (sParameterName[0] === sParam) {
  return sParameterName[1] === undefined ? true : sParameterName[1];
  }
  }
  };
  
  var companyid = getUrlParameter('companyid');
    
  $(function() {
  
  $('<custom field>').val(companyid);
    
  });
  
  
  });
  
  
  
</script>