Merge pull request #77848 from jaszperez/patch-47

update note on custom domains use

Author: PRMerger14

articles/active-directory/manage-apps/application-proxy-configure-single-sign-on-on-premises-apps.md

@@ -54,7 +54,7 @@ Keep in mind the following when you're going through the tutorial:
      ![Enter basic SAML configuration data](./media/application-proxy-configure-single-sign-on-on-premises-apps/basic-saml-configuration.png)
 
     > [!NOTE]
-    > If the backend application expects the **Reply URL** to be the internal URL, you'll need to install the My Apps secure sign-in extension on users' devices. This extension will automatically redirect to the appropriate Application Proxy Service. To install the extension, see [My Apps secure sign-in extension](../user-help/my-apps-portal-end-user-access.md#download-and-install-the-my-apps-secure-sign-in-extension).
+    > If the backend application expects the **Reply URL** to be the internal URL, you'll need either use [custom domains](application-proxy-configure-custom-domain.md) to have matching internal and external URLS or install the My Apps secure sign-in extension on users' devices. This extension will automatically redirect to the appropriate Application Proxy Service. To install the extension, see [My Apps secure sign-in extension](../user-help/my-apps-portal-end-user-access.md#download-and-install-the-my-apps-secure-sign-in-extension).
 
 ## Test your app