Create a Promotion Rule

curl --request POST \
  --url http://{defaultHost}/api/v2/platform/promotion_rules \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "promotion_rule": {
    "promotion_id": "22",
    "type": "Spree::Promotion::Rules::Country"
  }
}'

{
  "data": {
    "id": "5",
    "type": "promotion_rule",
    "attributes": {
      "type": null,
      "created_at": "2022-11-08T19:35:01.131Z",
      "updated_at": "2022-11-08T19:35:01.131Z",
      "code": null,
      "preferences": {}
    },
    "relationships": {
      "promotion": {
        "data": {
          "id": "16",
          "type": "promotion"
        }
      }
    }
  }
}