This option allows the Form Page Part to collect data based on the query string (TODO: add link once published). This is a great way to collect conversion data for a campaign. This works well with the option to set a form field as Hidden.
Follow the steps below to set a form field to populate the default value from a query string.
1. Select a form field to open up the field details.
2. Enter the query string key from which to populate the form field default value