providers/proxy: add option to override host header with property mappings (#14927)

website/docs/add-secure-apps/providers/proxy/index.md

@@ -152,3 +152,17 @@ return {
 ```
 
 Afterwards, edit the _Proxy provider_ and add this new mapping. The expression is only evaluated when the user logs into the application.
+
+## Host header:ak-version[2025.6.1]
+
+By default, the proxy provider will use forwarded Host header received from the client. Starting with authentik 2025.6.1, it is possible to dynamically adjust the Host header with a property mapping.
+
+```python
+return {
+    "ak_proxy": {
+        "host_header": "my-internal-host-header"
+    }
+}
+```
+
+Afterwards, edit the _Proxy provider_ and add this new mapping. The expression is only evaluated when the user logs into the application.