Re: Adding Lease line
If I'm reading your post correctly, you want to understand how to configure something to be accessible via this special WAN connection only.
Where is the application that you want to make accessible? Is it in your LAN, or can it be on its own interface connected to the firewall?
It sounds like you're going to need to do some policy-based routing to get it to send packets back out on this special WAN if normally it goes out your current WAN. It might not even be possible depending on who all needs to access it or what it needs to be able to access...