curl -X POST \
 /billing-groups \
 --header "Content-Type: application/json" \
 --header "Authorization: Bearer <token>" \
 --data '{
    "customer": "ad3f4b3c-1b2a-4d5c-8e6f-7a8b9c0d1e2f",
    "name": "Billing of payments at the end of the month",
    "type": "end_of_month",
    "customDay": "31",
    "customMonth": "12"
}'

{
  "id": "01F9ZQZJZJQZJZJZJZJZJZJZJZ",
  "name": "Billing of payments at the end of the month",
  "type": "<any>",
  "customDay": 31,
  "customMonth": 12
}