Document x-default extension for API playground

api-playground/openapi-setup.mdx

@@ -4,7 +4,7 @@
 keywords: ["OpenAPI", "API specification", "Swagger"]
 ---
 
 OpenAPI is a specification for describing APIs. Mintlify supports OpenAPI 3.0 and 3.1 documents to generate interactive API documentation and keep it up to date.
 
 ## Add an OpenAPI specification file
 
@@ -60,7 +60,7 @@
 
 ### Specify the base URL for your API
 
 To enable the API playground, add a `servers` field to your OpenAPI specification with your API's base URL.
 
 ```json
 {
@@ -128,6 +128,29 @@
 
 For more information on defining and applying authentication, see [Authentication](https://swagger.io/docs/specification/authentication/) in the OpenAPI documentation.
 
+#### Set default values for security schemes
+
+Use the `x-default` extension on a security scheme to pre-fill the authentication field in the API playground. This is useful for providing placeholder values or test credentials that help users get started quickly.
+
+```json {6}
+{
+  "components": {
+    "securitySchemes": {
+      "apiKey": {
+        "type": "apiKey",
+        "in": "header",
+        "name": "X-API-Key",
+        "x-default": "your-api-key-here"
+      }
+    }
+  }
+}
+```
+
+The `x-default` extension is supported on `apiKey` and `http` bearer security scheme types. The value appears as the default input in the playground's authentication fields.
+
+You can also use `x-default` on any schema property in your OpenAPI specification to set a default value in the API playground without affecting the `default` field in the schema definition.
+
 ## Customize your endpoint pages
 
 Customize your endpoint pages by adding the `x-mint` extension to your OpenAPI specification. The `x-mint` extension gives you additional control over how your API documentation generates and displays.
@@ -419,7 +442,7 @@
 
 The method and path must exactly match your OpenAPI spec. If you have multiple OpenAPI specifications, include the path to the correct specification in your reference. You can also reference external OpenAPI URLs in `docs.json`.
 
 #### Autogenerate endpoint pages
 
 To autogenerate MDX files from your OpenAPI specification, use the Mintlify [scraper](https://www.npmjs.com/package/@mintlify/scraping).
 
@@ -469,9 +492,9 @@
 
 </CodeGroup>
 
 ## Webhooks
 
 Webhooks are HTTP callbacks that your API sends to notify external systems when events occur. Webhooks are supported in OpenAPI 3.1+ documents.
 
 Add a `webhooks` field to your OpenAPI document alongside the `paths` field.