KNO-10168: Document default Content-Type header for fetch and webhook steps

content/designing-workflows/fetch-function.mdx

@@ -36,6 +36,14 @@ Aside from the request method selector, each of the above fields is a Liquid-com
 
 See the [Knock template editor reference](/send-notifications/designing-workflows/template-editor) for detailed information on working with Liquid templates in Knock.
 
+### Default headers
+
+When a fetch function includes a request body, Knock automatically adds a `Content-Type: application/json` header to the request. This ensures your endpoint receives properly formatted JSON data with the correct content type.
+
+You can override this default by explicitly setting your own `Content-Type` header in the headers field. This allows you to use alternative content types (such as `application/xml` or `text/plain`) when needed for your specific use case.
+
+For `GET` requests or requests without a body, no default `Content-Type` header is added.
+
 ## Request execution
 
 When executing the request for a fetch function, Knock expects the following from your service:

content/integrations/webhook/overview.mdx

@@ -69,6 +69,14 @@ To learn how to build a webhook request in detail, you can read [our fetch funct
   }
 />
 
+### Default headers
+
+When a webhook request includes a request body, Knock automatically adds a `Content-Type: application/json` header to the request. This ensures your endpoint receives properly formatted JSON data with the correct content type.
+
+You can override this default by explicitly setting your own `Content-Type` header in the headers field. This allows you to use alternative content types when needed for your specific use case.
+
+For `GET` requests or requests without a body, no default `Content-Type` header is added.
+
 ## Use your webhook channel
 
 Once your webhook channel is configured and you've built its webhook request, you're ready to add your webhook channel to a workflow.