After creating list and adding / generating codes to list, you still need to apply list to WPForms 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 WPforms form edit page
And you'll see the BSK Blacklist block in Advanced tab.
You may customize the validation message that to be shown in front, if leave it blank then the validation message in form settings will be shown.
Choose action for list
Invitation code list only have an action of Allow, it means only the field value same as one code in the selected list can pass validation.
Purchase BSK Forms Blacklist Pro