/**
    * MODADC - Get and decode the selected product for the form autopopulation
    */
   function getProductDetails() {
      if(isset($_GET['product_item'])) {
         return urldecode($_GET['product_item']);
      } else {
         return '';
      }
   }

// Call the function that gets the GET var
      $product_item = $this->getProductDetails();
      
      // Only inject the javascript if we have the required var
      if($product_item != '') {
      
         Requirements::customScript(<<<JS
            // This function adds functions to the onload event, so all the orignal javascript still works.
            // No changes needed in this function at all
            function addLoadEvent(func) {
               var oldonload = window.onload;
               if (typeof window.onload != 'function') {
                  window.onload = func;
               } else {
                  window.onload = function() {
                     if (oldonload) {
                        oldonload();
                     }
                     func();
                  }
               }
            }
            
            function setDefaults() {
               // I used firebug to get the id of the field on the form I want to target
               // Use firebug to do the same in your case
               if(document.getElementById('Form_Form_EditableTextField3')) {
                  document.getElementById('Form_Form_EditableTextField3').value = '$product_item';
               }
            }
            
            // Add the setDefaults function to the onload event, so it is only called after the page loads
            addLoadEvent(setDefaults);
   
JS
         );
      
      }