The address should be autogenerated. When you check the Allow blogging checkbox in the Blogs settings Sitefinity will generate the appropriate HTML headers. Please make sure the Sitefinity user that you use to connect with Live Writer is not currently logged in in the Backend on some browser. There is a known bug that prevents you properly setting up Live Writer if the user is already logged in.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items