After creating list and adding keywords to list, 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
You may customize the validation message for field, if you leave it blank then the validation message will use what you set in form settings page.
For "Select comparison..." dropdown, it is the way how to check the field value match a field. If includes same or contain( field value contain keyword in list ) with case-insensitive or not.
Update Formidable Forms form to save what you set to the field.
Purchase BSK Forms Blacklist Pro