After creating list and adding / generating invitation codes you still need to apply list to Formidable Forms form field.
Before apply invitation codes list to a form field you need enable BSK Blacklist on form settings page.
Focus on the field you'd like to apply list to on Formidable forms form edit page
Choose action for list
Invitation Code List only have one action of Allow, it means only the field value same as one of codes in the selected list that can pass validation.
Update Formidable Forms form to save what you set to the field.
Purchase BSK Forms Blacklist Pro