> ## Documentation Index > Fetch the complete documentation index at: https://docs.supercycle.com/llms.txt > Use this file to discover all available pages before exploring further. # Create a custom field > Create a custom field on an item or rental. Specify the definition either by definitionId, or by key and ownerType. ## OpenAPI ````yaml https://app.supercycle.com/docs/openapi-v1.yml post /custom_fields openapi: 3.1.0 info: title: Supercycle API version: 1.0.0 contact: name: Supercycle Support email: support@supercycle.com description: >+ The Supercycle API provides comprehensive endpoints designed to streamline inventory, rental, and return management for merchants using the Supercycle platform. This API facilitates the integration with external systems such as Shopify, enabling seamless synchronization of product data, rental operations, and returns processing. Key features include: - **Inventory Management:** Endpoints to create, update, and retrieve detailed information about items and products. - **Rental Operations:** Manage rentals from creation to dispatch, with automated serial allocation and synchronization with Shopify orders. - **Returns Processing:** Efficiently manage and track returns with the ability to update statuses and item conditions. - **Timeline Comments:** Add comments to timeline events associated with resources in a shop. - **Availability Timelines:** Per-variant day-by-day availability counts for back-office and OMS integrations (mirrors the storefront availability timeline). - **Blocked Dates:** List, retrieve, and create manually blocked date ranges on items, variants, and products that reduce availability. **Rate limits:** Requests are throttled per API key (Bearer token). Across all endpoints, a key may make at most **120 requests per minute**. The availability timeline endpoint (`GET /availability_timelines`) is limited to **10 requests per minute** per key because it is computationally expensive—cache responses client-side where possible. Requests without a Bearer token are limited to **30 requests per minute** per client IP. When exceeded, the API returns **429 Too Many Requests** with `Retry-After`, `X-RateLimit-Limit`, `X-RateLimit-Remaining`, and `X-RateLimit-Reset` headers. This API is essential for merchants looking to enhance their operational efficiency, providing the tools needed to manage their inventory, rentals, returns, timeline events, availability planning, and blocked dates all in one place. For support or enquiries, please contact [Supercycle Support](mailto:support@supercycle.com). servers: - url: https://app.supercycle.com/api/v1 security: - bearerAuth: [] tags: - name: AvailabilityTimelines - name: BlockedDates - name: Conditions - name: CustomFieldDefinitions - name: CustomFields - name: Items - name: Products - name: Variants - name: Rentals - name: ReturnOrders - name: TimelineComments - name: Locations paths: /custom_fields: post: tags: - CustomFields summary: Create a custom field description: >- Create a custom field on an item or rental. Specify the definition either by definitionId, or by key and ownerType. operationId: postCustomField requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/CustomFieldCreate' responses: '201': description: Custom field created successfully content: application/json: schema: $ref: '#/components/schemas/CustomField' '422': $ref: '#/components/responses/UnprocessableEntity' '429': $ref: '#/components/responses/TooManyRequests' components: schemas: CustomFieldCreate: type: object required: - owner_id - value properties: definition_id: type: integer format: int64 description: >- ID of the custom field definition. Required if key/owner_type not provided. key: type: string description: >- Key of the custom field definition. Required with owner_type if definition_id not provided. owner_type: type: string enum: - item - rental description: >- Type of owner resource. Required with key if definition_id not provided. owner_id: type: integer format: int64 description: ID of the item or rental to attach the custom field to. value: type: string description: Value of the custom field. CustomField: type: object required: - id - ownerId - ownerType - definitionId - key - value properties: id: type: integer format: int64 description: Numeric ID of the custom field. ownerId: type: integer format: int64 description: ID of the item or rental that owns this custom field. ownerType: type: string enum: - Item - Rental description: Type of the owner resource. definitionId: type: integer format: int64 description: ID of the custom field definition. key: type: string description: Key of the custom field definition. value: type: string description: Raw stored value of the custom field. valueJson: description: >- Typed JSON value of the custom field (format depends on definition type). responses: UnprocessableEntity: description: Invalid request content: application/json: schema: type: object properties: errors: type: array items: type: string example: Invalid request body TooManyRequests: description: Rate limit exceeded headers: Retry-After: description: Seconds until the current throttle window resets. schema: type: integer example: 42 X-RateLimit-Limit: description: Maximum requests allowed in the window that triggered this response. schema: type: integer example: 120 X-RateLimit-Remaining: description: >- Remaining requests in the window (always 0 when this response is returned). schema: type: integer example: 0 X-RateLimit-Reset: description: ISO 8601 timestamp when the throttle window resets. schema: type: string format: date-time content: application/json: schema: type: object required: - error properties: error: type: string example: Rate limit exceeded. Retry after 42 seconds. securitySchemes: bearerAuth: type: http scheme: bearer ````