The following example will relate the newly created submission to another element (in this case a Craft Entry) via the Freeform Submission relation element field type.
In order to view this page live, you'll need to make some adjustments to this template or on your test from:
freeformRelateSubmission or adjust the form handle name inside this template to match your test form.freeform-test and includes the following Craft fields at minimum, or adjust the values inside this template:
productReviews and the Sources setting include the freeformRelateSubmission form.