[Rules] Add example for subrequests from other zones (#23065)

Author: pedrosousa

src/content/docs/rules/transform/examples/add-request-header-subrequest-other-zone.mdx

@@ -0,0 +1,32 @@
+---
+pcx_content_type: example
+summary: Create a request header transform rule to add an HTTP header when the Workers subrequest comes from a different zone.
+products:
+  - Transform Rules
+operation:
+  - Request modification
+title: Add a request header for subrequests from other zones
+description: Create a request header transform rule to add an HTTP header when the Workers subrequest comes from a different zone.
+---
+
+import { Example } from "~/components";
+
+The following request header transform rule adds an HTTP header to Workers subrequests coming from a different zone:
+
+<Example>
+
+Text in **Expression Editor** (replace `myappexample.com` with your domain):
+
+```txt
+(cf.worker.upstream_zone != "" and cf.worker.upstream_zone != "myappexample.com")
+```
+
+Selected operation under **Modify request header**: _Set static_
+
+**Header name**: `X-External-Workers-Subrequest`
+
+**Value**: `1`
+
+</Example>
+
+The [`cf.worker.upstream_zone`](/ruleset-engine/rules-language/fields/reference/cf.worker.upstream_zone/) field used in the rule expression is set to empty if the current request is not a Workers subrequest.