Since Sitefinity 9.0, the FormEntriesSeed property is deprecated as noted in the API breaking changes. This blog post will provide the correct way to submit a form entry using the latest API.
For versions below Sitefinity 9.0, check official Sitefinity documentation to create form responses.
For Sitefinity 9.0+ use the code below:
Note the usage of GetNextRefferalCode method, part of the FormManager's Provider class. The method uses the entry type and returns proper referral code.