Front door with private link service

Has anyone successfully used frontdoor with private link service? 

I have a typical setup that a VM with only private interface running IIS. In the same subnet as the VM, I created an internal load balancer. In the Front Door (Premium), I created the site and the origin has the private link service enabled, and approved. However, I can't reach to the site through frontdoor no matter what, though I can hit the load balancer directly and show the page without issue. One question I have is, in the frontdoor origin --> Host Name, what do you use there? Is that the private IP of the load balancer or the frontdoor url or the custom url for the site? Can't seem to find a clear document that has some details on.