docs: add library example for HTTP API

site/docs/providers/http.md

@@ -40,6 +40,8 @@ tests:
 
 If not specified, HTTP POST with content-type application/json is assumed.
 
+## Parsing a JSON response
+
 If your API responds with a JSON object and you want to pick out a specific value, use the `responseParser` property to set a Javascript snippet that manipulates the provided `json` object. 
 
 For example, this `responseParser` configuration:
@@ -79,4 +81,27 @@ Extracts the message content from this response:
  }
  ]
 }
+```
+
+## Using as a library
+
+If you are using promptfoo as a [node library](/docs/usage/node-package/), you can provide the equivalent provider config:
+
+```js
+{
+ // ...
+ providers: [{
+ id: 'https://example.com/generate',
+ config: {
+ method: 'POST',
+ headers: {
+ 'Content-Type': 'application/json',
+ },
+ body: {
+ foo: '{{bar}}',
+ },
+ responseParser: (json) => json.output,
+ }
+ }],
+}
 ```