You've pointed guest ip in wrong manner it should be ip of guest (if you're statically assign ip address to guest) or leave it empty. Note you're pointed host ip as 127.0.0.1 it means that port you're forwarding will be accessible only from port (if it isn't what you're desired it should be empty as well).
VBoxManage modifyvm "win" --natpf1 ",tcp,,8081,,8081"
perhaps this is something you've wanted to do. (please look here for more details)