curl --request POST \
  --url https://api.metronome.com/v1/contracts/amend \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "customer_id": "13117714-3f05-48e5-a6e9-a66093f13b4d",
  "contract_id": "d7abd0cd-4ae9-4db7-8676-e986a4ebd8dc",
  "starting_at": "2020-01-01T00:00:00.000Z"
}'

{
  "data": {
    "id": "d7abd0cd-4ae9-4db7-8676-e986a4ebd8dc"
  }
}