What is the topic path in microsoft service bus?

Topic path is the same string that was used as topic's name to create it. See examples here: https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-dotnet-how-to-use-topics-subscriptions

Note ASB names are subject to some limitations: https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-quotas