Why can't Azure Traffic Manager be used with Service Bus?

I am trying to use Traffic Manager to front 2 Service Buses in different regions to provide resiliency without having to send duplicate messages to both regions, as described at https://learn.microsoft.com/en-us/azure/service-bus-messaging/service-bus-outages-disasters

I was hoping to use the Service Bus REST API to make Traffic Manager query the URL namespace.servicebus.windows.net/ for it's health check, since it will return 200 normally and presumably will fail to connect if a region faces an outage.

Since the drop down in Traffic Manager refuses to let me pick an arbitrary URL when "Azure Endpoint" is selected, I tried "External Endpoint" with the above URL, but the endpoint will not create. It fails with "Failed to save configuration changes to Traffic Manager profile Error: The domain name xxx is invalid".

If the url xxx.servicebus.windows.net/ can be resolved externally, why can't Traffic Manager allow this?

When adding the endpoint with the External Endpoint type, you need to add the Fully-qualified domain name (FQDN) as follows:

Note: A FQDN is a domain name that includes a host name, a root domain and a top level domain. It may also include additional subdomains between the root domain and the host name. For more details, you could refer to here.