curl --request POST \
  --url https://api-eu.flexportal.io/v1/billing-groups \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'Tenant-ID: <tenant-id>' \
  --data '
{
  "customerId": "<string>",
  "groupName": "<string>",
  "billingDay": 14,
  "rentalIds": [
    "<string>"
  ],
  "notes": "<string>"
}
'

{
  "success": true,
  "message": "<string>",
  "billingGroup": {
    "billingGroupId": "<string>",
    "tenantId": "<string>",
    "customerId": "<string>",
    "groupName": "<string>",
    "billingDay": 14,
    "totalMonthlyAmount": 1,
    "currency": "<string>",
    "status": "active",
    "createdBy": "<string>",
    "createdAt": "<string>",
    "updatedAt": "<string>",
    "rentalIds": [],
    "activeRentalCount": 0,
    "notes": "<string>"
  }
}