The API does not use the form configuration.
I just did a quick test with the payload below and received the email receipt.
{
"email": "justatest@test.com",
"enabled": true,
"expire_time": "0",
"password": "aruba123",
"role_id": 2,
"username": "justatest@test.com",
"visitor_company": "The Empire",
"visitor_name": "Darth Vader",
"auto_send_smtp": 1
}