curl --request PUT \
  --url https://api.loopreturns.com/api/v1/collections \
  --header 'Content-Type: application/json' \
  --header 'X-Authorization: <api-key>' \
  --data '
{
  "external_id": "<string>",
  "name": "<string>",
  "channel": "<string>",
  "products": [
    {
      "id": 123
    }
  ]
}
'

{
  "id": 123,
  "external_id": "<string>",
  "channel": "<string>",
  "name": "<string>",
  "products": [
    {
      "id": 2,
      "external_id": "<string>",
      "sku": "<string>",
      "name": "<string>"
    }
  ]
}