Update custom-domains-certificates.md

Add a note with a link to additional steps for internal Container Apps environments.

Author: gabesmsft

articles/container-apps/custom-domains-certificates.md

@@ -59,6 +59,9 @@ Azure Container Apps allows you to bind one or more custom domains to a containe
 
 Once the add operation is complete, you see your domain name in the list of custom domains.
 
+> [!NOTE]
+> For container apps in internal Container Apps environments, [additional configuration](./networking.md#dns) is required to use custom domains with VNET-scope ingress.
+
 ## Managing certificates
 
 You can manage certificates via the Container Apps environment or through an individual container app.
@@ -84,4 +87,4 @@ You can manage your certificates for an individual domain name by selecting the
 ## Next steps
 
 > [!div class="nextstepaction"]
-> [Authentication in Azure Container Apps](authentication.md)
+> [Authentication in Azure Container Apps](authentication.md)