Add missing firewall rule instructions for AuthHost debugging with Fiddler

Co-authored-by: alvinashcraft <[email protected]>

Author: Copilot

uwp/security/web-authentication-broker.md

@@ -178,3 +178,25 @@ The Fiddler web debugger can be used with apps. For more information, see [Fiddl
     ```
 
 3.  Add a firewall rule for incoming traffic to Fiddler.
+
+    You can create the firewall rule using either the command line or the Windows Defender Firewall GUI:
+
+    **Option A: Using netsh command (run as administrator):**
+    ```cmd
+    netsh advfirewall firewall add rule name="Fiddler for AuthHost Debugging" dir=in action=allow program="C:\Program Files\Fiddler\Fiddler.exe" enable=yes
+    ```
+    
+    If Fiddler is installed in a different location, adjust the path accordingly. Alternatively, you can create a port-based rule:
+    ```cmd
+    netsh advfirewall firewall add rule name="Fiddler Port for AuthHost" dir=in action=allow protocol=TCP localport=8888
+    ```
+
+    **Option B: Using Windows Defender Firewall with Advanced Security:**
+    1. Open Windows Defender Firewall with Advanced Security (run `wf.msc`)
+    2. Click "Inbound Rules" in the left pane
+    3. Click "New Rule..." in the right pane
+    4. Select "Program" and click Next
+    5. Browse to your Fiddler executable (typically `C:\Program Files\Fiddler\Fiddler.exe`) and click Next
+    6. Select "Allow the connection" and click Next
+    7. Check all network types (Domain, Private, Public) and click Next
+    8. Give the rule a name like "Fiddler for AuthHost Debugging" and click Finish