Funnels & Websites values in the address bar after the ? and merge those in. Here's an example:
When you select these checkboxes, it will add the custom fields to the end of the ?
Problems will occur if you have too many custom fields or your custom fields are too long. You'll get a long address like this:
If it becomes too long, the browser will break and it won't load the page.
The solution is to manually add your custom fields to the address like this:
Note that we DO NOT have the boxes automatically add contact data to the URL checked. Now it will only merge the data that you specifically ask it to merge and add into the URL.