curl --request POST \
  --url https://app.flowglad.com/api/v1/subscriptions/{id}/preview-adjust \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "adjustment": {
    "timing": "<string>",
    "newSubscriptionItems": [
      {
        "subscriptionId": "<string>",
        "addedDate": 0,
        "unitPrice": 4503599627370495,
        "quantity": 4503599627370495,
        "type": "<string>",
        "name": "<string>",
        "priceId": "<string>",
        "metadata": {},
        "externalId": "<string>",
        "expiredAt": 0,
        "manuallyCreated": true
      }
    ],
    "prorateCurrentBillingPeriod": true
  }
}
'

{
  "canAdjust": true,
  "previewGeneratedAt": 0,
  "reason": "<string>",
  "prorationAmount": 123,
  "currentPlanTotal": 123,
  "newPlanTotal": 123,
  "resolvedTiming": "immediately",
  "effectiveDate": 0,
  "isUpgrade": true,
  "percentThroughBillingPeriod": 123,
  "billingPeriodEnd": 0,
  "paymentMethod": {
    "id": "<string>",
    "type": "<string>",
    "last4": "<string>",
    "brand": "<string>"
  },
  "currentSubscriptionItems": [
    {
      "name": "<string>",
      "unitPrice": 123,
      "quantity": 123,
      "priceId": "<string>"
    }
  ],
  "newSubscriptionItems": [
    {
      "name": "<string>",
      "unitPrice": 123,
      "quantity": 123,
      "priceId": "<string>"
    }
  ]
}