const options = {
  method: 'POST',
  headers: {
    'x-spree-api-key': '<api-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    items: [{fulfillment_item_id: '<string>', quantity: 123}],
    reason_id: '<string>',
    memo: '<string>'
  })
};

fetch('http://{defaultHost}/api/v3/store/orders/{order_id}/returns', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request POST \
  --url http://{defaultHost}/api/v3/store/orders/{order_id}/returns \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'x-spree-api-key: <api-key>' \
  --data '
{
  "items": [
    {
      "fulfillment_item_id": "<string>",
      "quantity": 123
    }
  ],
  "reason_id": "<string>",
  "memo": "<string>"
}
'

{
  "id": "ret_gbHJdmfrXB",
  "number": "RET1002",
  "status": "requested",
  "order_id": "or_gbHJdmfrXB",
  "reason_id": "rar_UkLWZg9DAJ",
  "refund_total": "10.00",
  "display_refund_total": "$10.00",
  "refund_tax_total": "0.00",
  "display_refund_tax_total": "$0.00",
  "approved_at": null,
  "received_at": null,
  "refunded_at": null,
  "canceled_at": null
}